904 to 500 swap, 74 Dart, how much work?

You will need to substantially modify the center crossmember. The A500 is wider in the rear section. And no, you can't just cut away the crossmember; the torsion bars also anchor to it so it carries the weight of the front end.

Not a bolt in swap, but if you're halfway handy with a welder, it shouldn't be too hard.