I never have seen a nv3500 have a failure death. I have seen several of them and even several 4500 with syncros that were out between 2nd and 3rd. If you slowed your shift it would be fine, but never have I seen on leave someone stranded. I am not saying it would be a good or bad choice for the 6.2 but I think it is a better unit than you give it credit for.
MOST have bad syncros..
my cousins doesnt even have a syncro left in it.. you gotta shift REALLY SLOW..
if you bolt a 3550 up to your 6.2, i wish you luck..
Another thing to take into consideration is the weight, the nv3500 only came in light vehicles that weren't geared towards towing duties... I don't know exactly how you drive or use your truck.
a buddy of mine has a nv4500 behind a 350 chevy, with practically no syncros...lots of fun riding with him, revving the engine all over the place grinding the gears like mad trying to get into the next gear, he doesn't seem to understand the concept of shifting a non-syncro transmission.
Just to be clear again, I have the 3 5 5 0 Not a 3 5 0 0. I have an NV4500 in my 2500HD chevy. The PO put 90wt in it and 2nd always grinds now, other than that I have towed and worked it a lot and it takes a beating *step 1 putting syncromesh into it*.
In all the jeep forums I have read they say that the ax15 is maybe good for 10hp more than the NV3550. They have stated that both are good trans for a 4.0L jeep or a mildly built small block. I couldn't find an ax15 and I found the NV for about as much as a rebuild kit and it came out of a 39,000 mile jeep. So that is why I am doing it this way. I personally think the NV 4500 is quite a trans and hopefully the 3550 will be similar for this wrangler that is 1/3 the weight of my truck and less power too.