Chris and Calvin have it about right on the higher HP. 310's. If they are rebuilt by someone who knows the engine they will last a good many hours. We sold several 1955's and not one of the original engines made it to much over 2,000 hours. Some of em a lot less. The rebuids generally lasted a lot longer. The really sad part about it is that the 1955 would flat-out hands down work circles around anything in it's class. Chris, correct me if I am wrong on this, but wasn't the 1755/1855/1955 chassis designed to handle around 120 H.P.? I don't know what a 5.9 Cummins puts out, but if you keep it to around 120 or a tad less you should never have any trouble.