My bet is the splines on the input shaft extension are starting to get worn and it wobbles from the slop. If the clutch was out of balance the vibration would be constant although more pronounced at higher rpm.
I've driven NV4500's before and thought they shifted just fine, keeping in mind they aren't supposed to compare to a short throw narrow ratio box. The negative experiences above could be related to worn out tranny's or maybe goofy geometry on the shifter because of the bends required to clear the heaters, etc.
My philosophy is the fewer the adapters the better off you are. If you can get rid of some adapters because the tranny mates up wit the motor and live with the way it shifts, I would go that route. I would also seriously consider the TH400 automatic suggested above. Meanie has that same tranny with an Atlas T/C. It's the shizh because there are no adapters, it's bullet proof strong, gives you the excellent crawl ratios because of the torque convertor and the 4-1 ratio in the Atlas. We've done all of the hardest and steepest trails in Moab and Penrose with it and I've come to be a believer in the automatics performance offroad. Ours has a performance valve body in it and with the 4-1 TC it stays locked up when crawling down the steep stuff so you get as much engine hold back as with a stick. And, I don't care how good of a driver you are, you can't out shift a properly setup automatic when you're lead footing it in the snow.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang