John
If H pipes are properly cut, fit, and welds are properly reinforced along with the use of proper hangers they will last for years. I ran a government fleet for many years and never had a problem. The main reason that ford came out with them in the early sixties was to reduce noise it had nothing to do with emmissions or power. I used to drive a big block Ford with Straight pipes and no mufflers and never got a ticket for noise. The pipes had a "BALANCE TUBE" as ford called, it located just to the rear of the transmission mount. I have found that most pipe breakage problems come from improper mounting. A system has to be mounted so that it can flex with the contrasting movment of the engine and chassis.
If they are mounted to rigid something will break, coming back to ford they was having trouble with flange leaks/breakage at the convertor then they started using rubber supports hung from fixed posts on body and pipes and solved most of the problem, or at least slowed it down. You will also find flex joints used on semis to allow for movement between the cab, chassis, and engine. I have seen 5" pipe break in the middle of a straight and unwelded run when mounted to solid.
