v-6 in early A-body

I completely disagree with 98% of your post. ... never had a complaint about vibration. Third, your comment about loosing crank trigger is WRONG. ... There is some cross member modification to fit a 44/46 RE ... have you ever done the swap? Have you ever owned a magnum engine?
98% disagreement sounds like bad math. My comments were just possible issues for the OP to read-up on. You gave much useful info for everyone, stop fuming.
A 60 deg bank angle V-6 is much more ideal. The V-6 Magnums used the same angle as V-8's, to keep factory tooling easier. The consensus I have read is that the V-6 Magnum was a temporary solution. Perhaps it continued longer because there were extras to use up.
If that crank sensor bracket you show is factory in Magnum engines, that makes it easy. I assumed one had to fab a bracket to work on a 904 bell-housing. "No", I have never owned a Magnum engine, only picked parts from them at a junkyard.
Custom-fabb'ing a new transmission tunnel in the car is a big bite for most, but does give a better transmission, and if the floor is rusted out it could be a two-fer solution.
While the 3.8L is a more optimal V-6 design, it won't bolt-up to a 904 (I assume), so would require using a Jeep 2WD transmission w/ probably the same custom tunnel mod's, plus one would be pioneering the engine controls. Is there something inherently different about the 3.8L that makes it a low performer? Mine has a modern intake w/ long runners, pushrods like a Magnum (and Chevy LS), but a coil-pack which is better than a Magnum but worse than current "coil on plug" designs.

The 3.8L "toner ring" slots are much different. Rather than 1 per spark, equally-spaced like the Magnum, it is a coded pattern of unequally spaced slots. It also varied over the years, which I found when I got the wrong one (~2003 w/ ~24 slots) for my 2002 (12 slots). The flex-plates suffered cracking (mine, see photo). Like the Magnum, the toner ring is welded to the flex-plate, which also has the starter ring teeth. The crank sensor is a shorter 3-wire Hall-effect type that bolts to a pad on my FWD tranny, don't know how they mount in a Jeep.

PICT0007.JPG