I am swapping a 455 block into a 72 GTO that had a 400 in it. Are the 400 mounts on the frame different from the 455 mounts?

400 mounts r the same to 455, pontiac never changed the mounts for the different engine options that generation ...however i hope ur looking into getting some stonger mounts, 575hp is in territory for new rear end, tranny?

yes they are the same , you have an automatic tranny ? headers are a real pain with the 4 speed & i had to relocate the oil filter & some interferance issues with the Z bar with hooker headers.

if the motor mounts are old you will lose some clearance, now mounts will help