[Richard]

The stock front axle in a K20 would have been the Dana 44. Later trucks, Suburbans and Blazers/Tahoe's would use a corporate 10 bolt. All the way into the 90's. As you are adding the axle year does not matter if complete. There are slight differences. You could step up to the Dana 60 which was used in one tons. The passenger drop D60's are getting really hard to find. I am having a new one built to my specs for various reasons. Do not think it is really necessary for your application. Stronger is always better, your call. Looking at what you have and want to do. If it was mine I would do coilovers in the back with air assist bags for load handling. Would require some custom fab work. You are probably going to need some custom fabbed coils for the rear in any case.
More leafs does not mean a better ride, sometimes just for load. Many thin leafs equal better ride, more spring movement and longer life. Sometimes the individual leafs are different thickness. That is what you get with the higher end spring manufacture's