I am not sure what the fram looks like under the 64, but I bet that a 4x4 setup could be put under it. You could, if you wanted to, use coil springs with a solid axle. To do this you would need to build yourself a multi-link suspension. If you want an idea of what that would look like look under a jeep wrangler next time you see one, or a dodge 1/2 ton pickup from the body style previous to the current one. Essentially you would have two lower "links" and two upper "links" it would be pretty trick, and they are awesome in off raod conditions.
For the mild off roader you would probably want to get yourself a Dana 44 front axle, these were made in both 6 and 8 lug patterns. A good transfer case to look for is the NP-205 which was a GM factory piece on many 4x4's and shouldn't be too hard to find. You said that you have a 4speed, is it one for a 4x4? If not I would suggest the SM465, a good tough 4spd.
You can look around online for info on building a custom 4 link front suspension. You could also look at your local magazine store and find a couple of 4x4 magazines to leaf through for ideas and parts.
I am in agreement with the other guys when they say that it will be easier to drop your truck body on a different chassis. I just realized that your rear suspension will probably need some drastic modification to. You could definitely make your truck and its chassis into a 4x4, but I wonder if you would rather just change it over and have the benefits of the newer chassis.
As far as lift I would go with a 6" lift and a 35" or 36" tire. This will give you a good all around ability off the road while keeping the truck reasonably streetable. It is also a great looking size for a pickup.
Keep posting and I will keep talkin your ear off!
Kyle
