Pathfinder Conversions made some 4x4 vans...
There was a few GM vans around here that were converted to 4x4's by Pathfinder Industries in CA,they used normal K5 or K10 front ends with leaf springs,and a NP205 T-case--the ones I saw were TH350 tranny's,not sure if any manual tranny's were offered..
I have an 81 G10 I considered converting a while back,when I wanted to swap in a V8,and I had a rolling K10 chassis out in the back yard..
but I decided it was a lot of work I'd rather not do,and the van was starting to get rusty,so I never pursued the project..
The vans are either 110" or 125" wheelbases,depending on whether its the "shorty" version or not...so a K5's 109.5 wheelbase IS real close to the short ones wheelbase--a K10 with an 8' bed is 131.5 inches,a lilltle too long,but you would not be using the original "K" series frame anyway,only adding the 4x4 drivetrain to the vans "unirails".....I know the motor is offset in my van towards the passengers side quite a bit,and sits slightly slanted instead of paralell with the unirails,but I think that doesn't really affect anything,the driveshaft appears to be "slanted" too,rather than a straight shot to the diff,and it does not affect a stock van..
A van rear axle is much wider than a K5 or pickup truck one too--might be best to just use the vans original rear axle,and install a matching ratio front 4x4 axle..could be fun joining the steering colum to a 4x4 front axle..

My van has a "straight" colum,right into the box,not hard to adapt--the later "G" series have a "driveshaft" with 2 U-joints at goofy angles,and the box stands straight up and down,they made it "collapsible" so the steering colum wont impale the driver in a crash--could be a bit tricky to hook that setup to a 4x4 axle..
It would be best to find one already converted to look at--I've seen several "homebuilt" ones using ford axles and divorced transfer cases,and they looked simpler than the GM drivetrains to install and make work...
