Hopefully this info I'll give will help you out, cuz I already know being in PA, you're in big trouble already just trying to find a K series truck that's not half rusted away. Then on top of that, wanting a K20, makes it even harder yet since the K10's are probably 3x more available than K20's.
So, I'm doing the same thing with my K10/20 project. It's originally a K10 but I'm converting it to 3/4 ton springs and axles. My conversion in my opinion, will even be better than a K20. Here's why. K20 trucks come with 52in springs like the K10 springs, just stiffer and stronger. The C20's come with 56in springs. Stronger spings, but longer so should offer a better ride.
The long bed K10 frames are punched to use either 52in or 56in springs. So I'll be drilling the rivets to the rear spring hangers and moving them back 4in to the other set up holes put there by the factory. Then installing the 56in C20 2wd 56in 3/4 ton C6P springs. Should give me a little more lift, heavier spring capacity and yet keeping a smoother ride with the longer springs.
Then I'm using a 14b Full Float axle out of 85 C20. The front springs, I'm using Rancho 4in lift springs for K10/K20. I can use the same front axle, I just have to swap the 6 lug hubs and brakes for 8 lug brakes and hubs.
Then use a K20 driveshaft or use a conversion U Joint and you'd all the powertrain of a K20 in a K10. In some states, it's better to do it that way since the higher the GVWR, the higher the annual tag is. Cali is one of those states for example. Though in Cali, higher GVWR means it can run dirtier too and still pass smog. So pros and cons either way, but it's not hard to convert a K10 to K20.
