Could be the clutches or bands. Might recommend Kevlar if you have to dig in anyway. A 2400 stall should have a minimal affect on mileage. I ran a 2400 in my coupe, for about a year and never had a problem. If you go racing which I see you do, I would also recommend a Manual Valve body, reverse pattern. Maybe an Art Carr. I run a 4000 stall in my FB, lauch at 4600 against the 3 step, so the HP actually overcomes the stall. Once you have the reverse pattern, you will never go back. Much quicker.
Ron
65 FB is at
www.rfedd.bigstep.com