I swapped a TH350 into a '65 Chevelle that originally had a Powerglide. The length is exactly the same, it used the same mounts and the driveshaft needed no modification. Just make sure you get the standard TH350 from a car, there are some truck trannys that are longer if I remember correctly.
I would also recommend making new tranny cooler lines, you can bend your own fairly inexpensive. I'm not sure if you could even reuse the old ones, I didn't even try since I set up a remote cooler when I did the swap and needed new lines anyways. You can go to Auto Zone, Advance, ect.. and get the unbent lines. I know Advances part number for a 6ft line with 5/16" fittings is BL560. They call it a Brake Line, but it's the same thing.
The "hardest" part was modifying the shift linkage. Basically all I did was go to the junkyard and find a car with a TH family transmission. I ended up using a Oldsmobile with a 200-4R, it's similar enough. I got all the lower brackets from under the car that act as the pivot to change the gears. The pretty much bolted on where the stock powerglide brackets did, the only change I needed to make was one rod that was about 8 inches long needed to be 6 inches, I just cut it and welded it back together.
And that was it, the speedometer cable just hooks right on and your done. Total cost not including the transmission is $5 each for the Brake Lines, and $6 for the junkyard shift linkage assy. Like I said I did it on a Chevelle, but I really don't think it would go much different on your car. Good luck!
