In my experience, pitfalls of a V8:
1) increased interior heat from the larger displacement/power output/exhaust system (especially on long interstate drives)
2) harder to keep the engine cool when working it hard (I've yet to get a 258/4.0 to run Hot)
3) the fun/expense of swapping a V8 in (all the little details)
4) less MPG (I would "think" the newer generation GM V8's would get same/better then a MPI 4.2/4.0, depending on pedal usage)
5) running the exhaust, single or dual (clearance at ground/transmission/front driveshaft/transfer case/cross members/ rear shackles/etc.)
6) easier to break drive train components (depending on skinny pedal usage)
Obviously, lots of benefits to a V8 swap, too
I've had two Jeeps with V8's, a 304 and a TBI 350. Both were fine/fun to drive, I just prefer my inline 6's
As far as gearing goes, 4.10's and 33's with a 6 cylinder is good. BUT, depending on conditions, i.e. lots of mountain driving or driving across the great plains with strong head winds, 4.56 might be better. I ran 35's with 4.10's and a T176 behind a 6 cylinder. It was barely "OK".
IMO, 35's and a 6 cylinder, minimum of 4.56, I would probably go 4.88's
With a V8 you can be a hair less aggressive with the gearing, BUT keeping any engine in it's powerband is critical for peak performance