work truck use and stick shift = nv4500 and a round pattern 205 or 241 or my last choice 208 behind it all t-cases need to be of the 32 spline input gear .
work truck and auto with overdrive should be = 4l80-e and simple way to run tranny electrics is
www.usshift.com been around for years . the new unit design does 4 different shift programs . ( if you have the older one its a simple upgrade fee for anyone interested )
same t-case options as the nv4500.
your k20 if my my dads old one was sm465 4 speed with np 205 t-case and 4.56 gears .
stock 4.56 & 235/85/16 ( 31.7") = 3320 rpm @ 70mph
nv4500 swap = 2535 rpm @ 70mph
stock again with tall 285/75/16 ( 32.8") = only drops from 3320 rpm to 3267 rpm
nv4500 with 285/75/16 ( 32.8") = 2450 rpm @ 70mph
these rpm's will be the same with the 4l80-e also in overdrive as the 4500 share .75 5th ratio
even if you went to a 34.5ish tire aka 35" its 2341 rpm @ 70mph .
hope this helps .
play here for the info I gave
http://www.grimmjeeper.com/gears.html