The best thing to bolt on to a 3.9 is an engine stand, then put a 4.6 in its place.
I ran a crower 229 in my 3.9 (actually a 4.0 block) with all the 14cux stuff retained. It was ok, but the 4.6 it's got now is so much better. I can pass people on the highway without downshifting. Both were...