650+ , she'll hold up fine , I've put a quickchange in my last 4 builds n there still kickin. They don't like shock loads so a manual cummins would probably be out of the question though
1.5 .134 tube has the same load bearing ability a1 3/4 .120, I usually use 1 3/4 in my mud truck chassis but I wanted a smaller hotrod look with the same strength