Thanks, but I just reinstalled the bolts with the engine and transmission out of the car.I tightened them as much as possible until it spun the engine and used Permatex thread sealer gel. Couldn't get a torque wrench on them.Today is the first startup since.
I can't see any marks on the bolts, maybe I'll try painting the heads to see what's up.

Is there anything else it could be?

I ran a stock converter with 5/16 bolts and never had a problem.I haven't measured them both yet,but they look about the same.Tired and cold and done for the day, will check it out tomorrow.

Is there a set of bolts with a thinner head than the ARP's?