NV4500 will shift better IMO. Rebuilding the Sm465 with new syncros etc will help and be very close in feel. Others have mentioned this already but a lot depends on how you use your 40. If it's mainly a road rig and sees highway regularly, the overdrive of the NV will be a nice feature. If it rarely sees speeds that would allow for the overdrive, then I would opt for the shorter drivetrain with the 465 and have better rear driveshaft angles. I had a SM420 in my 40 that I had completely rebuilt and it was smooth. Easily one of the best mods I did to the truck for offroad and I always drove it to trails so I was fine with it on the freeway too.