I'm looking at a similar 4x4 conversion. I've pretty much resigned myself to 8-lug due to availability. Keep in mind that these trucks only had about a 10,000 GVWR, which is about the same as a modern 3/4 ton truck. I think modern 1-ton axles should give more than adequate payload.
Also keep in mind that front springs on your truck are probably about 10" closer together than a modern truck which will definitely cause issues.
Last edited by kert; 01/04/2014 2:13 AM.