Back in the day - I seem to recall the issues with Boss 302 pistons cracking. I'm guessing that few manufacturers would honour a warranty once the car has been on the race track. I'm guessing it would have had other things changed, like headers etc. Interesting fact that I had the opportunity during the last couple of years to buy back the '71 429 SCJ Mustang I originally bought when it was 10 years old. Very original other than headers and wrong colour paint it only had 58K miles on it. This was probably 3K miles more than when I sold it 30 years ago. It had what sounded to me like a bad camshaft (supposedly solid lifter stock) but later turned out surprisingly to be a detached piston skirt (much like your Boss I guess?) - was your '69 Mustnag a 351 Windsor or Cleveland?
Hi Roscobbc
My '69 Mach I 351 CI had the Windsor heads, and they had the cooling flow issues where the Cleveland was supposed to have solved that problem
The issue was with the Boss 302 was half skirt pistons,
they would crack right where the skirt sides ended as they were a weak point for aluminum design back then :-(
I should have done is back in 1969 of all nameplates is Rambler had the AMX which had a sweet 390 CI and the manual 4 speed tranny that was a real bullet.
A buddy of mine bought one, and I drag raced it a few times, and it was a sleeper and no one thought a Rambler would have any balls