You say the rear is from a 75 and the engine a 71 but what year is the IFS you're going to use? Either of these years came with a 250 I6 so factory engine mounts are readily available. There is a good chance the factory mounts won't work for the needed radiator and fire wall clearance but since you will have to fabricate a core support mount on the new IFS frame clip you may just have to make up a new core support itself which you can position for needed clearances. We do this on AD to S10 chassis swaps and it's not a huge chore. If you don't have a Nova clip yet the 74 and older ones are rear steer and a bit easier to hook up steering. DO NOT USE a Camaro/Firebird clip on an AD.
