How many using an A500 overdrive ?

92b said
Are higher stall converters available in lockup version for the A500, say 3000 to 3500 range?

Yeah. The same converter that fits a 904 lockup trans will fit a 500 unless it's a newer 500 with the "flats" in the inner pump rotor instead of the slots. Don't remember when that change happened. Pump rotors can be changed between the 2 types