Buschi340 and I are doing the same trans swap in this thread, so you can go through that and get an idea of what's involved in making that trans fit. FWIW, the AX-15 Dakota trans has a slightly different shifter location, so that MIGHT be a bonus, but it's still a fat unit.

As for the V6 vs 273 question, the 273 is only 0.58L or 35ci larger, and has LOTS more potential for upgrades of any kind, plus they grew in those engine bays from the factory. Not wanting to discourage your efforts in doing something different, but strictly from a constructability standpoint, the 273 gets the nod in my book. Now, if you HAVE to do the 3.9, mate it up to an LA bellhousing & trans, bolt it in, and set up the engine mounts off of that assembly. There shouldn't be any issues using the LA bell on the 3.9, and an 833 would bolt right up if the crank is drilled deep enough.

Other interesting engines might be the 4.7L OHC jewel (I want to do one of those myself), or even the 3.7L OHC V6. Just try to keep all the factory EFI stuff if possible, and it might be fairly straight forward to install. LA bellhousings will fit both of those, too.

Clair