Try supporting the muffler with a metal retaining strap. It sounds like you are getting engine vibration that's causing the elbow to crack. I finally took my muffler off and ran a straight copper pipe for the exhaust. That stopped my elbow from cracking. I never could support the muffer because the vibration kept breaking the strap. A brass or copper exhaust is much lighter than the muffler. The noise difference is not noticable and since you don't need muffler pressure to the fuel tank, it might solve the problem.

pmw