Well I'll take a stab at this.I believe the 460 will fit in the 71-73 chassis and a C-6 is probably the tranny to use just have to ensure it has the correct bell housing. Also not sure about motor mounts for that chassis. Just my best guess here.
Yes, the 385 series will fit into the '71 - '73's. That's the 429/460. Since they were optional, you can get everything to bolt it all up. Trans I would go with a C6 if an auto, and a big shaft toploader 4 spd if manual.