Diesel A body

I've owned a few that smoked like Diesels......
I agree that a Cummins 5.9 would be extremely heavy and overkill. The engine and trans would require extensive bracing and reinforcements to resist torsional stress. The engine alone weighs over 1000 lbs! A fully dressed 440 weighs around 700.
The 4BT has been discussed before, and may actually be a decent idea. If you had access to a junkyard full of parts for free, this could be fun. Otherwise, you'd spend far more on this conversion than you would save in fuel. Its fun to think about though.....