OK- - - -pretty good diagnosis procedure! On a hard uphill pull, that pipe can get very close to red hot, especially if the carburetor is running a little lean. That's when a flex line might try to melt. I've seen that happen on many dyno runs on some of the race engines I've built. It's also possible to fabricate a heat shield between the flex line and the exhaust pipe but putting a 4 or 6 inch long pipe nipple between the block and the tee is the quickest, simplest fix to give the flex line a little more protection.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!