The NV3500 has a two piece case that splits into a front and back half. To convert a 2WD NV3500 to 4X4, you need to change the rear half of the case and the output shaft.
Easier to find a 4X4 trans to start with.
The NP231C is the stock case that comes with a 4.3 S-10. Get an 89' and up with the electric speedo hook up.
The easiest 4X4 conversion would use a Jeep reverse cut, high pinion Dana 30 front axle. I would use a Cherokee axle because it is cheaper and easier to find. You have to weld on spring perches as the Cherokee used coil springs up front. A TJ or YJ has leaf springs but they are harder to find.
Mount the axle with a hand made cross member and rear leaf springs from a full size Chevy half ton truck. I have drawings of the cross member if you would like to see them. Shackles mount to the frame on the rear part of the springs.
Use stock steering parts from the Cherokee as well. The pitman arm will bolt to the stock S-10 steering box, and give you a 4" drop pitman arm. Lift will be in the 4"-5" range over all. 5" will clear a 33-12.50.
The rear end lift is as simple as a spring over conversion. Cut the spring perches off and weld them on top. Bolt it back together with the axle under the springs. When you get the donor front axle, get the rear as well. It will save the hassle of gear ratio and different wheel bolt pattern woes.
Over all, the conversion isn't as hard as most think. There is, still quite a bit to it though. Little things like drive shaft length and shocks to work out. The Jeep used a NP231 so the switch to run the CAD front axle will screw right into the S-10 case. A drive shaft safety loop welded into the trans cross member will allow the front drive shaft to pass threw. You don't need an expensive conversion oil pan like you do with a conventional S-10 4X4 V-8 swap. You can also use the cheap Headman 2WD headers that won't fit a 4X4 chassis because of interference with shock mounts. Dan