Its not necessarily that the ealy 4V's made less torque that the later PI headed 2V's (they make slightly more actually) its that said torque comes on somewhat later in the power band by comparison. A few guys have experimented with modifying the IMRC motors to allow the plates to open a little later on in the power band that they're set up from the factory (approx 4,000-4,200RPM). This is said to improve torque, throttle response and in town fuel economy greatly accross the board with the early B-headed DOHC engines.
I might consider looking into that before going too drastic with the rear gears. Especially if this Mustang is going to spend any time on the highway and is expected to retain any sort of fuel economy.