It wasn't really scepticism, but in order to make a recommendation for clutches or transmission parts, I often ask for verification on engine horsepower. A lot of people are "told" they have so much, and others can produce a dyno sheet. Usually, the ones that are "told" don't have what they are "told".
Being an engine builder, I know how hard it would be to get over 700hp out of a 505ci FE while still making a lot more torque than horsepower without forced induction.
As for the transmission, TKO's just don't shift well at 6500 rpm. That's just a given. They'll fight to the end at that kind of RPMs. I think what's happened is that you may have bent a shifter fork at some point trying to get it into gear at a higher rpm.
There are ways to fix this, especially if a service center needs to go into the transmission anyway.
If you have a mechanic looking at it, then I will wait for verification of the diagnosis before I prescribe a remedy. If you would want to send it to me, I'd be happy to go through it.
So what clutch are you running?