IIRC, the blocks are identical, just a different crank and rods inside it. Should bolt in no problem.
Heck, you may not even have to swap the ECU at all, a lot of guys run the 3.9's with 4.2 ECU's.
The 14CUX is surprisingly good at how well it adapts.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.