Y'all missed the easy button. Find a 'K' TH400 case and swap the guts out of the AMC TH400 case. Nothing fancy there, and shouldn't be too difficult to find. Now he's got a Chevy bellhousing bolt pattern. I'll bet that the converter isn't odd or unique, so keep it too. Cost of a K case is way, way less than the rare AMC case that he's got. Possibly even enough so that it will offset the cost of having a pro do the swap of the trans guts.
Then a BBC or SBC bolts to it w/o issue, and an LS bolts to it with a small spacer of some sort (never done it myself).
While you're at it, get the t/c cover specific to the K case and the braces and engine mount plates. These will stiffen up that whole engine-trans assembly.
Really surprised that I'm the first to mention this.
As to what engine I'd bolt in front of it, that's easy. A TBI 350. The engine management is stupid simple (it's ECU is also not terribly 'smart' by modern stds but that doesn't matter), can get 300HP out of it w/o leaning on anything too hard, and the system's wiring and plumbing needs are also very simple. It is NOT fancy or modern or super-duper zippy, but it's easy to diagnose in the field, needed spares are limited, and it works well. My old TBI truck ran fine from sea level to 12,400' It didn't make sea level HP at 12.4, but neither did I!