YES, I was a Pontiac GURU at one time, hahaha...., all Pontiac Motors, 350- 455, the heads off one will bolt on another, all the blocks are the same, all except for bore and strokes. Waterpump off a 350 will bolt to a 400 or a 455, oilpans. EXCEPT for the HO's or the SuperDutys. Heads are completely different. Alot of the internal parts are the same also, except for the crank journal sizes.... gotta be careful there.
Great motors, reliable, torquey. Lots of speed equip. for them, though some of it can get expensive.
Rods are heavy, cast with the long stroke 5200-5500 max. Good steel rods and crank, will turn to 6500-7200 and higher.
The 389's and 421's won't interchange parts with the 350/400/455 family. Now the HO's and the SuperDutys are awesome! Big, huge ports, Steel rods and steel cranks....and are very hard to find. But well worth it if you fine one in a bone yard. Or a sale somewhere....if you pick one up cheap, your very fortunate.
I had a Early 70's Firebird, Had a 74' 455 SuperDuty, Turbo 400 with manual valvebody, 4.56's in a 12 bolt, and she'd run in the Mid 11's in the early 80's before I put a 8-71 on her. I drove that car on the street.