The original M3 Lee had the M2 75mm (derived from the French field gun). It was 84" long and required a counterweight so it could operate with the then revolutionary gyrostabilization system that kept the gun at a certain elevation while on the move. The gyro system was developed to work with the longer barreled M3 75mm gun which was 110" long. The barrel was lengthened to increase muzzle velocity, but since the tank was still considered an infantry support weapon and not an anti-tank weapon, the M2 was considered adequate and still used although it was being phased out in favor of the M3.

Bottom line, add the counterweight if you wish, but shorten the barrel if you do. BTW the Airfix 1/32 scale Lee and Grant kits contained the short barreled gun with counterweight. Hopefully someone will one day re-do a Lee/Grant in 1/35, much like the Stuart was redone. The Lee is my favorite tank.