2 Engine Swap Options - Advice?

[greywurm]

My '71 has a 250 in it with a small crack in the head around the temp sensor that leaks coolant slowly with a JB Weld patch job. I'm looking to swap back in a 350 (PO replaced original 350 with the 250).

I have a couple of local ads, both im pretty interested in, but am unsure about the first motor's performance. From some research on the block number, I think it is either the LT9 or LS9, the LT9 sounds like it isn't very good unless I were to change the heads and cams. Block number is 14010207, the seller says its from a 79, but the block number tells me its an 80. Anyway to tell whether it is the LT9 or LS9?

It's a complete motor minus the starter and carb, supposedly was rebuilt and sat for 8 years because he never got started on his project. He's selling it for $400, $500 if I want to grab the TH350, or $1500 if I also want the 79 truck which I dont really want.

An alternative is another 350, complete minus pulleys with edelbrock intake and carb, block number 3970010 with th400 for $600. A little further to drive though. Seller said mileage is about "5-7k weekends only to car shows local swamp meets was in my 57 pick up with turbo 400 with shift kit ran good, replaced with a 5.3, no pulleys used that set on other truck"

I think I might be leaning to the 3970010 block, especially with the better tranny and a better chance it isnt the under powered LT9. What are your thoughts on these?

Is it much work to change from a th350 to a th400?

Thanks all.

[bbcmudtruck]

I'd go with the second setup.

[slowcpe]

The story definitely sounds better on the second one. From what I'm hearing that's the one I'd go with.

[68C15]

rebuilt 8 years ago and has been sitting?! I'd pass on that one.

option 2 is my advice. It just sounds like a more honest guy and the block # is a good one to have.

[greywurm]

since response was unanimous and matched where i was leaning, went for 0010 block. Thanks guys. Now time to do some more reading up on doing swaps.... especially th350 to th400... first time swapping engines!

[68C15]

TH350-TH400 swap
1. new driveshaft (different size splines in back of trans)
2. move crossmember (simple, slide it in frame to match with trans)
3. install kick-down switch on gas pedal bracket and wire it up

[greywurm]

Originally Posted by 68C15

TH350-TH400 swap
1. new driveshaft (different size splines in back of trans)
2. move crossmember (simple, slide it in frame to match with trans)
3. install kick-down switch on gas pedal bracket and wire it up

I read about the first two points yesterday... Thinking about keeping the th350 in there until I get a spare shaft from the junk yard sized up.

But I haven't heard about the kick down switch. Is it electronic on the 400? Anything else that's different about it?

[MARKDTN]

#2 is a better deal. The TH400 has a switch for kickdown that is activated by the accelerator pedal. No big deal. It is a much more robust transmission than a TH350-although there is nothing wrong with a TH350.

[bbcmudtruck]

If there isn't anything wrong with your th350, i'd just use that one. The 400s are stronger, but I wouldn't see it being necessary on a mild or even a stout sbc motor. I have been debating whether or not I'm going to use my th400 or my th350 behind my big block build. The local trans guy that builds all the mud runners transmissions is telling me to stick with the th350. He will guarantee it live behind a 750+ hp bbc motor for an $800 dollar build price.

[greywurm]

Originally Posted by bbcmudtruck

If there isn't anything wrong with your th350, i'd just use that one.

I might have an issue with it, might not. I think there is an issue with the vacuum line and/or the vacuum on the monojet carb. Part of why I want to keep it in for now to see if its really the tranny or how the l6 was installed. The metal vacuum line doesn't quite reach the carb properly so I have a little more soft line than I would like.

[greywurm]

Picked up the engine and tranny Saturday. 3970010 block, edelbrock carb (600 cfm I think) and th400 tranny.

A few quick questions.... I am trying to get an attack plan together.


One of the pics, and circled a couple things on the tranny I think I have ID'd correctly: 1) post for the shift linkage 2) electronic kick down 3) speedo sensor. The guy had a digital dash, so the sensor is electronic. Can I reuse this or replace something to go back to a cable? I have a stock dash cluster.

Can anyone ID the exhaust headers? He said they were hookers, but I can find anything that looks like them. Are they worth keeping or should I get new ones before I size up the exhaust?

I got the transmission separated from the engine because I wanted to get the engine on a stand to clean up and paint. When I was looking up how to get the torque converter off (the bolts are on pretty solid), I came across a post with some comments talking about potentially screwing up the pump and/or seals on the front of the tranny by taking the engine off with the TC. Any way to tell if I screwed these up? They look ok from the outside.

I also was under the impression that the th400 was longer than the th350. I measured both and they both are about 28.5 inches long. Is that possible? From what I can tell, I might be able to swap out the tranny as well. The th400 has an improved shift kit installed, so it might be nice.

Can I reuse the torque converter that is on the new motor with the th350 if I dont swap?

[68C15]

You can use the TH350 trans and converter with the new motor. The TH400 takes a different converter than the TH350 but they both bolt up to flywheel the same. Your circled parts are correct. Get a cable driven speedo set up from the dealer. You'll need to remove the tail housing to install the drive gear on the tail shaft. Get your tire size and rear gear ratio before you go to the dealer. tH400 has a different slip yoke than a TH350 on the front of drive shaft.
I'm not sure on the headers, good luck with that

[mechanicalman]

I see no reason whatsoever to put the TH400 in. It would require the mods already mentioned, and there is no guarantee that it is in better shape than the trans you have.

Torque converter will interchange if you have a dual pattern flywheel. Most flywheels are dual pattern, but a few older ones only had one pattern. I'd prefer to use the TH350 torque converter regardless which trans you go with. But I'd go with the TH350 and it's own torque converter. The TH400 requires more horsepower to run than the TH350, the internals are heavier and more mass to get moving (lots of drag). TH400 will withstand more power, but I doubt you will come close to exceeding the TH350's power limits.

You might consider replacing the torque converter to front pump seal while the engine is out.

[greywurm]

Alright, I'll stick with the th350 that I have. Just need to figure out where it's leaks are coming from then. I have been suspecting the pan and the cooler lines.

I guess the best bet is to use the fly wheel and torque converter that I have in the 250/th350 setup instead of what is on the new 350 motor now? I didn't know if the TC's would be different... But if so, I just need to get the stubborn bolts holding the TC to the fly wheel off. Any tips on that? I am suspecting it was installed with thread locker. Its not like I can drill them out with a screw extractor...