a 5.9 magnum carb swap is easy...
-aftermarket intake manifold, some kind of 4 bbl carb.
-late 70's F/J 360 engine mounts or schumacher mounts.
-use the flexplate that came with the engine (it will be balanced to the engine) and use a 318 neutral-balanced torque convertor.
-if using the serpentine belt setup off the new engine, you will need a p/s pump and bracket off a full size v6/v8 b-van. Ram, dakota; durango and grand cherokee brackets are too long to fit our cars.
-alternator must be 97 or older to fit dippy wiring, or matched to harness if going EFI
-the manifolds that come with those engines are excellent, but will NOT fit our cars. i wish like hell they did... but you will not clear the firewall.
-if using factory transmission, you need to notch the bellhousing for the crank sensor (if using EFI) and in any case install a shift kit. Raising line pressure and quickening shifts is a great idea when adding this much more power. If you want to swap 904, the v6 B van transmission will bolt right into a diplomat, and have the crank sensor slot in it. Most rams, dakotas etc have 4 speed transmissions; alot of extra work and money to make it fit.
-upgrade the rear end. (8.25" or 8.75"
F-M-J bodies all interchange, and 69 and older B bodies fit with minimal differences. The stock 7.25 axle is really good for stock non-abusive, under 150hp use. There is no other drop-in swap options for these cars, correct me if i'm wrong.
-There is alot of other details, but that covers the broader bases. I just finished this swap on my 81 coupe...