Why not just swap over the spring hangers from the K 10 frame, bolt them onto the K 20 frame. Use grade 8 hardware. They don't have to be riveted. In my opinion bolts are better. Semi frames are bolted together and they take allot more torque / abuse than any 3/4, or 1 ton can dish out. I have bolted every cross member and bracket into several Chevy frames and a couple Fords too. They held up just fine.