Rambler American engine swap
It looks like you have been sent quite a few ideas about possible swaps. I have a '59 American wagon project car that runs on 5 of the flathead's 6 cylinders. Due to the narrow engine compartment, I had been thinking in terms of a more recent and powerful inline 6. Unfortunately, most everything I looked at was a much longer engine, so I would have to cut the firewall to make it fit.
One possibility I am considering is a major transplant from a '78-'83 280ZX (though I'm sure plugging a Japanese drivetrain into an American will make some of my buddies shudder). The beauty of this swap is that the 280ZX had front and rear subframes, the 2+2 model was within an inch of the same wheelbase as the Rambler, it would have discs all around, much improved suspension, and ZXs are readily available and cheap.
The down side is that I don't have the equipment or skills needed to do it myself so I would need to find a creative and trustworthy shop and the firewall would definitely need to be cut. In theory, with front and rear subframes, it should be fairly straightforward to swap engine, transmission, suspension and the works.
Alas, my '69 Fairlane's 302 croaked, so my car budget is currently covering a rebuild. Best of luck with your project.