[Big Dave]

An engine swap under your belt makes you a bona fide hot-rodder.

A 6.2 or 6.5 will be an easier swap than a 4BT3.9, but the latter may be more satisfactory in the long run.

Having 300,000 miles with a 6.2 I recommend that if you have the sand for an engine swap, go whole hog and do a transmission swap. diesels are so right with manuals and so wrong with automatics. My 6.2 destroyed seven automatics including two TH400s. Not just "it won't shift" but spectacular failures scattering burnt ATF and blue-colored parts down the road. GM offered a NV4500 in thiose trucks about that time so a five-speed should bolt right up to a 6.2 or 6.5.

There is a whole website devoted to 4BT3.9 swaps.