You should swap in a 2.5L 4-banger just to piss off
@'68cuda416 and other people like him. We don't need that kind of negativity in this hobby. 2.5L is a good engine and it's 100% MOPAR, I've heard of guys pushing 6-7-800 HP on those with basically stock bottom ends. I know of at least one guy who has swapped a 3.6L Pentastar in an A-body and it kicks ***, runs 13s in the quarter and gets 25+ MPG. It's not easy though due to the exhaust manifold being integrated into the head it doesn't leave much clearance for exhaust in older cars. My mom drives a 2014 Chrysler 300 AWD with the 3.6L and that thing hauls for being a 4400-lb full-size sedan (yes I know the 8-speed helps a lot but it still moves even at high speed).
If it was mine I'd swap in a 5.2L Magnum in a heartbeat, keep the EFI if you can manage just for an extra couple MPG and ease of use. I like the low-gear-set 904/999 idea too, I have 2.94 gears in my Duster and they're perfect for highway cruising. That combo in an A-body should easily
average 20 MPG or more.