I have access to a free th400. How difficult is it to swap the st300 that's in my 68 cutlass with the th400? The th400 is also coming out of a chevy. Will that make any difference or is a th400 a th400? My cutlass has the rocket 350 in it.

 

OK, first, the TH400 was factory available in the 1968 A-body cars. That's the good news. The bad news is that a Chevy trans has a different bellhousing bolt pattern than your Olds and won't bolt up to the engine. Also, the TH400 came in three different tailhousing lenghts. The A-body cars used the shortest one. Since short tailhousing Chevy TH400s are sought after by the Chevelle crowd, I doubt that's what this is.

 

I just read that there's a chevy to bop adapter plate. I'm not sure about the tailhousing length. It's coming out of an early 70's vette

 

Are you using it because iit's cherry ..?

 

In which case I'd sell it to a Corvette restorer, buy the correct Olds trans, and pocket the difference.

 

If you have to modify anything to make it fit like shorten the drive shaft and buy an adapter plate then the transmission is really not free. Make sure of what you facing before you start down this path.

 

If all I end up having to do is buy the adapter plate for $60 bucks than I'm going to do it. If there's much more that I have to do other than that I'll keep looking. I figure 60 bucks for a newly rebuilt transmission isn't a bad deal.

 

Rich, don't forget you will need to shorten your drive shaft and get the slip yoke for the T-400. You may need a trans mount for the 400 may be different.
Like "Joe" said sell the Chevy trans and buy the correct one, or get a T-350 and it will bolt right in. Same drive shaft, same mount. Still has to be BOP pattern.
Gene

 

Why do you want to replace the transmission you have? For $100 you can buy a shift kit and filter from Summit for your existing transmission. Buy it and slap it in there on the weekend. Seems like you would have more trouble adapting the Chevy transmission. Does the current owner of the Chevy transmission have a receipt showing that he had it rebuilt recently?

 

Custom driveshaft length and different front yoke for the driveshaft. Add those costs. If the car is a floor shift, you also need the correct TH400 bracket for the shift cable. And of course, there will be the different speedo drive gears.