When I replaced the frame on my CJ7, I went with new motor mounts because the old ones (over 20 years old at the time), and used polyurethane ones. That was 8 or 9 years ago, and haven't had any problems.

I'm not sure that motor mounts is the cause of your headers cracking. Usually, that problem is related to having the exhaust system tied solidly to the frame somewhere, rather than having flexible exhaust system mounts.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.