Hope you are considering my advice as helpfull not as a way to talk you out of it, I am trying to make sure you know what is going to be needed before hand.

If 6 cylinder power is all you need the v6 will certainly make more, the heads can make 300 HP NA without touching them, the slant, not so much, perhaps 200. There are probably more intake out there to pick from with the slant but the v6 manifold can have the runners cut down just like you do in the v8 manifold. You can get some thick phenoilc resin and make your own plenum plate to keep heat out of the air from the oil. Bore the holes out for a V8 throttle body and port the TB. The heads can be ported and bigger valves just like the V8 magnum.

The 3.3 and 3.8 are gonna be harder to get a good manifold as you cant really get in it to port without cuttin it open. No RWD trans that I know of. Performance pistons are gonna be totally custom, so are rods. The V6 magnum can use anything the 318 can.

I'm with you on this one! Go for the 3.9. Turbo would be fun.
I have a 3.9 with a 5 speed in my 48 Plymouth business coupe, and I like it very much. That early A body is probably lighter then my coupe. Adding a turbo might be more fun then I already have with it. I expect a build thread with the turbo 3.9.....

My coupe was built with a 90 3.9, but it has been replaced with a 91 3.9, both were throttle body injection and ODBI which is very easy to deal with. My 93 Dakota has a Magnum 3.9 with an auto (4x4) and it has port injection (barrel intake) and ODBII, its a little more complex to deal with, but I can feel the difference in power between the two motors, the magnum is hands down more powerful then the non-magnum. The magnum is backed by a OD auto, in a standard cab, long box, 4x4, with around 155K, the 91 (and the 90) are in a shortened Dakota frame (both wheel base, and trimmed off rear at the rear spring hangers) with a steel 48 Plymouth business coupe body, caring all the Dakota cab & box floors, and interior. The 91 motor has 118K miles and is backed by a 5 speed. Both vehicles have 3:55 rear gears (coupe is a limited slip, the truck is open) & run 235 x 75 R !5 tires.
The exhaust pipe ends on the 93 exhaust manifolds (rear dump) are a lot bigger then the exhaust pipe ends on the 90 or the 91 exhaust manifolds.

What year is your 3.9? Gene

All the 92-93 manifolds are bigger, Jeep, V6 and V8, I have all threee types here, 94 they shrank them to between the 91 and 92 styles. If you want to try some of the big V6 ones let me know as they are for sale.

It sounds like a good idea but it won't be an easy swap.
For less effort and heartache you could drop in a similar power 318 and be miles ahead.

The 3.9 isn't that efficient.

If you've got your heart set on this swap then get all over it. let us know how the swap goes and feel free to ask for input along the way.

Better look again, I repeat THERE ARE NO NORMAL MOUNT EARS like the style common on every other LA engine, only the flat bolts to the side of the block. Since the block is now shorter the mounts would have to occupy the same space on the side of the block. I have 3 at my shop right now, 2 magnums and 1 TBI an NO motor mount ears on either side of any of them.