Just to add to the "it doesn't fit" discussion regarding "V" motors--I recently mocked up an Olds 215 block (similar dimensions to the Buick V6) in a '60 American engine bay and.....surprise, it didn't fit. As others have stated, side clearances can probably be handled with some cutting, but the stock steering is the biggest problem.
Farna and Ram-- do you know if anyone has ever run a carbed 22R? I bought my American for an LS1/T56 swap, but am looking at a possible "temporary" motor after I set up the Mustang II. Just a thought at this point.