I would go another way with it. I would buy a Chevy with a V/8 and automatic and have a shop switch it over, probably $3K for that. I wouldn't disassemble the entire car, I would have the body and paint done, replace bad chrome and trim for $6K and redo the interior $2K and maybe $2 to $3K for odds and ends. That is $14K and that is iffy and still too much money. I used to know a guy Mr Chevrolet he was called, he would put that car together for maybe $5K, that is the type of person who needs to buy this car, otherwise it just isn't
Due to the age of that car I'd expect to replace all the brake hoses, wheel cylinders, springs and shoes together with all the steering linkage and front suspension bushings. Finding a good used engine and transmission is not going to be that easy. Between the last time a small block Chevy engine was used if memory is correct about 99-2000 if you find one it's going to probably need a full rebuild together with changes necessary to adapt it to a carburetor from fuel injection, transmissions since about 1980 have had some degree of electronics, some requiring a standalone electronic control unit to properly shift. Personally a GM crate motor, ballpark price in long block trim (block, heads, timing cover, oil pan, valve covers) last time I knew was about 2k. Add in the missing brackets, pulleys, mounts, there's probably another thousand or more added to it. Plus getting the A/C working, nothing turns me off more than seeing a factory equipped car with a non functioning half missing system. For the transmission, I'd externally reseal that Powerglide, drop the pan and change the filter and fluid at minimum. Maximum, have it gone completely through.
Body and paint prices, last car I had done was in 1996. Labor, paint and materials was about 4k, minimal rust, mostly dents. Before the car went in I replaced the deck lid and right door with better ones that I found. The body shop was owned by a long time friend, he used it as fill in work during slow times.
I still stand by my 20k estimate and I too agree that is way too much to put into that car, I can only see a return of about 13 to 15k when completed.