why am I breaking starters?

[DetroitDan]

I got this truck 2 years ago with a new starter and a bad flexplate. Replaced the flexplate and didn't drive it much for a year while I replaced other stuff. Few weeks ago the still new-looking starter failed, the stud that the battery cable goes on snapped the plastic or ceramic or whatever it is that it goes through. I know better than to overtighten the nut on that stud, but I thought maybe it was my fault and threw another starter in.

-I should mention that when I did the flexplate, I noticed the starter wiring was burned up and sketchy-looking, so I made a new harness up with new primary wire and fusible links, and put in a 1.0 gauge battery cable.-

I've been driving it around, to work and so forth with no problem. Last week I took it on it's maiden voyage towing the big camper, made it ten miles and with no warning the starter engaged. Pulled over and unhooked the battery, messed around with the wires and got it going again. Another ten miles it happened again, so I dropped the starter and found the same thing had happened, the stud was broken in the starter and the case part was cracked apart. I know damned well I didn't overtighten it this time. Got a ride to the nearest parts store, $25 and a few minutes work and I was underway again. I can't figure out how any torque could be getting put on it to break it like that. These are just cheap parts store starters, but they shouldn't keep failing in the same place for no reason.

Only thing that I can possibly think of is the new battery cable is heavier, and is kind of hanging loose. idk if maybe it could be bouncing enough to hurt something. Doesn't seem likely at all. I'm also real concerned about the heat coming from the exhaust manifold, it was starting to melt the new battery cable, even from a couple inches away. I was able to clock the cable down and tighten it so it's even farther from the manifold, but if it breaks again it will move and touch the unused starter terminal. Wonder if maybe the extreme heat alone is causing the solenoid cover to snap?

How do you get the heat out of these trucks? I know the 454 is known for cracking manifolds due to heat, but my engine temp (coolant) has stayed within a safe range.