Zenith,
Any 350, 400 or 455 built from '71 through '75, with 5 mounting holes drilled and tapped on each sid of the block will be a direct bolt-in. The earlier stuff can be more powerful without a lot of work, but a couple of small changes needed to make them bolt in. "The Book" explains most of it in good detail.
Don't over-think this. It isn't NEAR as difficult as putting a non-Pontiac (of any description) in there.
Jim
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