[Barry-ct]

There are aftermarket 3.73 ring and pinions, but they were not stock in a F-1, The 4 speed crash box tranny you have was an option. The measiest way is to swap the rear as suggested above. If you do the tranny swap, you'll have to swap driveshafts, tranny, and possibly the crossmember. I vote for the rear swap. Remember, these trucks were not built for speed, but to work.