More info to what Big Dave said: I have swapped transmissions in a few 60's Chevies. Consider an OD trans for awesome driveability. Since a 200R4 is a bit longer to the mount you have to get an aftermarket crossmember or weld two more brackets to the frame. The Powerglide has a face-to-mount length of 20 9/16". The TH350 is 20 3/8 so you need to tweak the trans mount a bit. And, a 200R4 is the same length but the mount is further back, 26 7/8." You have to make and weld an additional support bracket for the crossmember. I installed a BowTie Overdrive in a 66 Chevelle with original 283 and it gets over 20mpg now and it will even chirp the tires if that interests you.
My tech manual page won't load, but the new frame bracket is about 2-1/8 further back from the original. Some 66 frames had dual mounts from the factory, usually big block cars with a TH400 or all Caprice models.