It depends on what your definition of "tough as nails" is.
My first IDI was an 86 6.9 with a c6. When the engine locked up I put a 7.3 with a hypermax kit and the c6 never complained pulling a 10k trailer. Guy I sold the trans to is still running it in a plow truck and can't believe how good it shifts. I put 30k miles on that truck and who knows how many was on it before that.
I tore up 2 back in my younger days. Junk yard unit I put behind a solid running 351w. A dozen passes at the drag strip with one foot on the gas and the other on the brake in one night burnt it out. Actually it was the truck pull the next night that finished it. Still made it home 70 miles and it wouldn't move with less than 3000rpms. Had a reputable guy rebuild it and I said make it stout.
$800 later I had a solid rebuilt c6. Built a 408w stroker pushing 450hp/450ft lbs and it held up great behind it. 80mph at 6000 RPMS in 2nd gear and it would all but snap your neck when you jammed 3rd.
I often wonder why I could never keep that engine in 1 piece
better question is how I'm still alive.
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