If you're looking at a six popper, why not go for a 232, 258, or even a 4.0L swap? I've got a 1969 American and I'm working on putting a 4.0L head onto my 232. A 232 can be a 300hp engine without too much difficulty. Ask around and you'll find some members that still remember the Henry J's with 232's and triple webber carbs
If you're still hellbent on non-AMC power, oddrodder would be able to help you out, he's got an awesome rambler with a chevy 327 in it.