I have an '80 K10, with a new motor, put in a long time ago, the auto trans went, so I got creative summer of '94 and put a 4 spd from what I believe was a farm truck that rusted out at 38,000 miles, got complete set up, bought a new heavy duty clutch, I can't recall which NP transfer case it has 203 or 205, cast iron, gear drive, I had Spicer hubs on the front, liked the part time for fuel savings, the oils were clear in trans and transfer case, I saw no wear inside. I did all the u-joints, what a beast in the woods, I could line up 3"-4" poplar, birch, etc. with the snow plow frame, and run right over them, of course do not ever back up while doing this, low gear, just creep along. That old truck has been through blizzards, plowed a lot of snow in its time, pulled a tractor trailer, and a tandem wrecker from the pavement, both were slipping in my driveway, it looked good, little extra pull in low gear 4wd low. Its been in creek beds when there was a full moon, and it's cleared perimeters of fields.
The funniest thing I ever did with that truck was drive it up the nys thruway under lets just say questionable circumstances, lacking a few credentials. I had to drive it out of NJ from a yard at work, where I did this conversion. It has 270 gears, speedo was now off 10 mph, what a trip, big thunderstorm on the horizon. That old truck moved to and back home from NJ, been sitting awhile, one tough drivetrain. Some of the deep snow and hills I've been in, semi stuck in snow with it rubbing on the rocker panels, yet I could always get through.
