Nice thing about the 3.9 swap is that it puts the back of the engine about 4 inches closer to the radiator, putting most of the fat part of the trans tail shaft in an area of the trans tunnel that has more room.

I suspect the lesser weight and better aerodynamics will allow an A body to get better mileage than a truck. Don't forget, in stock form, the 3.9 is tuned to run richer than optimal for emissions reasons, the cats need fuel to work. If you kept it EFI but used Megasquirt to run it you could tune for better mileage.