L6 250ci with 3 Spd Manual to Big Block Conversion-Does trans need to be moved?? Help

[Yamariv]

I've been researching old threads for days about what's needed to convert to a bib block. Engine stands, bigger rad etc but I have found a few posts relating to standards transmissions that say:

1. you need a different cross member to support the standard trans
2. and you need the large hump tunnel cover cause the trans moves back a couple inches.

Is this actually needed cause I've found other posts saying swapping to a BB wasn't hard but they lacked these specific transmission details.

Thanks

[Accelo]

The 3 speed clutch and transmission are the same length as a Muncie, THM350, power-glide and 200/2004R.
Typically any/all can be interchanged without changing driveline length. Although some conversions will require the automatic cross-member.

I am wondering about the 3 speed behind the BBC. This was not a factory option because the transmission was not rated for BBC torque.

1. you need a different cross member to support the standard trans.
I don't see a need for changing the cross-member if you stay with the 3 speed.

2. you need the large hump tunnel cover cause the trans moves back a couple inches.
The large hump was behind BBC because they had the 4 speed manual or the THM 400.
If you are staying with the 3 speed no reason to change to the large hump.
If you use a 700R4/4L60 they fit under the standard floor.

FYI I have observed BBC installed on SBC mounts. This works but don't expect some issues like aftermarket headers will not fit.
Cheers.

[Yamariv]

Originally Posted by Accelo

The 3 speed clutch and transmission are the same length as a Muncie, THM350, power-glide and 200/2004R.
Typically any/all can be interchanged without changing driveline length. Although some conversions will require the automatic cross-member.

I am wondering about the 3 speed behind the BBC. This was not a factory option because the transmission was not rated for BBC torque.

1. you need a different cross member to support the standard trans.
I don't see a need for changing the cross-member if you stay with the 3 speed.

2. you need the large hump tunnel cover cause the trans moves back a couple inches.
The large hump was behind BBC because they had the 4 speed manual or the THM 400.
If you are staying with the 3 speed no reason to change to the large hump.
If you use a 700R4/4L60 they fit under the standard floor.

FYI I have observed BBC installed on SBC mounts. This works but don't expect some issues like aftermarket headers will not fit.
Cheers.

Thanks Accelo! I'm a bit confused though.. I plan on keeping the 3 speed. So I won't need the automatic crossmember? Any way to know for sure?

I guess the clarifying part I need is if the 3 speed standard will stay in the same spot as it is now with the BB engine stands? If I need a new cross member and have to move the tranny back then I think a BB is just too much work and cost. If it drops in with the 3 speed with no driveline changes then I'm gonna do it.

[kwmech]

You'll find long term that the 3 speed will complain about the big block in front of it from the added torque and horsepower. If you want to keep a manual transmission in the truck you may want to start sourcing a 5 speed that is up to the task instead of doing it twice. Big blocks ---in the stock form using factory frame stands----mount in the front holes on the frame and will push the engine forward 1-1/4'' of the small block position even though the stands bolt in the same holes. I 'm not sure of the 6 cyl placement vs big block as I have not converted from a 6 to a large V8

[Yamariv]

Originally Posted by kwmech

You'll find long term that the 3 speed will complain about the big block in front of it from the added torque and horsepower. If you want to keep a manual transmission in the truck you may want to start sourcing a 5 speed that is up to the task instead of doing it twice. Big blocks ---in the stock form using factory frame stands----mount in the front holes on the frame and will push the engine forward 1-1/4'' of the small block position even though the stands bolt in the same holes. I 'm not sure of the 6 cyl placement vs big block as I have not converted from a 6 to a large V8

I kept reading that the 3 Ott saginaw was the stock default tranny for the big block. The more I'm reading, I'm hearing that 3 Ott was not the syncro Saginaw, but a beefier tranny, is this true? If it was something beefier then I guess a big block is not in my future as I can't really afford to do a tranny swap as well.

I've read many posts saying the 3 Speed Syncro Saginaw is a very tough underrated transmission, if I go small block will it hold up fine if I had something like a 325hp -350lbs of torque mouse in there. That not too far off the 402's power numbers though..

[72SB]

The Saginaw 3 speed behind a SBC will be fine and will still hang from bell housing

IMHO a very streetable 400+ HP/TQ SBC will outperform an old school BB. Weigh less and run cooler and get 15mpg all day long

[Mickeymotormount]

Not sure on big blocks, but for the small block, the trans is moved forward since the engine is shorter in length than a 6 cylinder. This gets the fan closer to the radiator. Looking at the frame rail, there are 3 holes on each side for the engine mounts. The front 2 are used for V8s and the rear 2 for straight sixes.

My understanding of the V8s was that the big and small block share the same location for engine mounts. Hopefully someone can correct me if I am wrong.

Attached are a couple of pics showing the 6 cylinder and 8 cylinder bracket mounting positions on the frame.. I think the actual bracket may be slightly different between the 6 and 8 cylinder. The big blocks got a beefier bracket than the small blocks or sixes.

Notice how the 6 cylinder used the rear two holes while the V8 used the forward two holes.