But first you'll need to modify the transmission crossmember, change the front driveshaft yoke, possibly shorten the driveshaft (there were a few different 400 tailshafts), and get a new torque convertor. Also, the downshift on a 400 is controlled by an electrical solenoid so you'll need a downshift switch mounted either on the carb linkage or the gas pedal.
you could blow off the kick down, get a slap stick that way you don't have to wait for it. I have a 77 and just picked up my driveshaft that I had to have shortened 1 3/4". When I looked into getting a new yoke there is only one for it and it is the big one and you driveshaft will have the conversion to chevrolet joint. They run around 65 bucks aftermarket and be careful that the u joint eyes arent weak looking. If they don't look good and beefy they will break! Had to relocate the crossmember to the frame, pull the mount ear off, grind to fit and reweld it to the other side of the member. The only thing was really that easy about it is typing about it here. Don't have any pics (bad juju) but after I get done with the works I might take some.
The 400 is a way better choice. You will loose a little time in the 1/8th and 1/4 though. Its heaver and takes a little more from the power plant to get it moving but all in all it is still a way better choice if you're going to use a 3 speed auto.
The 400 is almost bulletproof but takes more horsepower to turn it. The th350 is strong enough for the small block with little or no modification. Th350's will bolt right up and unless you got more than 450hp, I'd save the 400 for a big block. Just my opinion
I was always told & it was dyno proven in one of the car mags that it takes 20 more horsepower to run a th400 over a th350. I have seen serious power though a th350 with no problems. On the street the th350 is a far better choice.