3 speed vs 4 speed auto tranny

[Sstarkey]

Got a '72 K5 CST with a regular 3 speed automatic coupled to a 350 engine. Any easy way to install a 4 speed automatic or something with an overdrive? Tks.

[SeventyOne]

"Easy" is a subjective term. It's been done and continues to be a popular swap. Try a search and you'll get a ton of info.

For a quick rundown: 700r4 is your best bet IMHO, you will need to get a new output shaft for the trans to mate up to your tcase, or an adapter, or swap tcases all together. Then due to the extra length of the 4spd vs 3spd you will need to move the engine forward or the tcase back. Both have pros and cons.

Good luck.

[Sstarkey]

Thanks for the direction on the swap from a 350 to 700 tranny - got plenty of space to move engine forward but just need to work out the cross-member. Everything else looks pretty.

[red71cheyenne]

I don't know about the 4X4 versions, but on the 2wd 700R4's, if you get a short tail shaft version, it will mount in the same place as the TH350. Thats what I did in the 71. Might be able to find the same thing in 4X, but someone with more knowledge will have to chime in.

[Firebirdjones]

It's a rather pricey swap on a 4x4.

I went with a 4L60E (electronic 700R4). I needed the shorter output shaft, and an adapter for the stock 203 transfer case.
Actually it was a little more in depth for me, as I needed the coupler and decided on a new transfer case mount as well because I didn't want to modify the original. The stock coupler doesn't work if you are using the ball bearing type. The swap kit is designed for the newer style splined sleeve that GM started using on later models.

All of the necessary parts are sold through Advance Adapters. I spent about $800 for everything to adapt (not including the transmission).

It can be done cheaper, depending on the route you go.

I chose to move the engine forward, as there was more than enough room for it. Simply redrill new holes in the frame for the crossmember.

[Sstarkey]

Thanks Firebird, I appreciate it - Did you get the cross member from Advance Adapters as well? I can't seem to find a cross member for the engine, only for transmissions and/or transfer cases. Am I missing something?

[Firebirdjones]

Sorry, I was under the impression that you were still using the original 350 engine in the blazer.
In which case you can just use the stock cross member and move it forward with the engine and redrill the frame. I don't believe there would be any clearance issues with the differential on that one. Might have to trial fit though and see. If you have a suspension lift then no worries.

I did an LS swap at the same time, so I used a custom cross member from Pacific Fab.