I have driven a few NV4500s. One was in a relatively new Chevy pickup. I found it to be not unlike a SM465 in that you had to accurately hit the slot with the shifter on the first try or getting it into gear was a PITA. I also drove a 60 that had a Vortec and the NV4500, nice motor, lame tranny. I felt like I was flexing some part of the shift linkage to get it to shift. It just didn't have that solid feeling of completing a shift that you get from an H42. I haven't driven an NV4500 offroad but my on road experience leads me to believe that it would be downright frustrating to drive on a trail where making a quick shift is very important. I'd rebuild the H55 and try it.
The vibration I suspect might be due to the Mark's adapter.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln