so should I be able to cut out the inner bumper frames and weld mounting blocks in?
Without crawling under your car and look at it; I 'll have to take a guess. I imagine you could cut out much of the support structure and keep the parts that mount the bumper to the frame. It depends on how the support structure is attached to the outer bumper. I'd box the ends of the supports you keep.
Time to crawl under and look around. I spent quite a lot of time under the bumper of the 73 figuring things out.
My friend just got rid of a '74 442. Had the Olds 350 and swivel seats. It was rear ended by a box truck several years ago, he has the swivel seats now is his '69 442 he bought with the insurance money.