I disagree with your assumption that the timing is 'dead on'. If you had a factory cam, a factory dizzy, and set the initial to the factory specs, then it would be dead on. But you don't have any of those. at best you are ballpark.
And not only that, but the later distributor you have, the worse the curve was for mileage. But at this point you (the OP) doesn't even know if that is working OK. If the dist has been replaced by a "rebuilt" in it's lifetime, anything "could be in there."
"Factory" timing is "not the best" for mileage OR performance!!!!