Well as a 40 gmc owner, I can understand your woes. The 3 speed is actually an eazier transmission to drive than the 4 speed. the 4 speeds had no syncros, so it required double shifting everything. My guess is the prom is sometime this month, and it may not be practical to drive that truck to it.

If he absolutely must drive it to the prom, fill it up with gear oil, and make sure the AAA membership is current. If and When the transmission finally gives up the ghost, you can call "Buffalo Steve" to make an adapter to bolt a t 5 transmission to your bellhousing. You will have to change or modify the rear at the same time for an open driveline.

Others have used the T5s and like them