just put the reverse and idler gear from a 4 spd in there..
I used to toss 4-speed transmissions. Now when i get them, I split them apart for the reverse gears, and toss the rest of the internals. The bell housings make a great, lightweight too for setting the timing on engines on the floor of your shop, and a way to test compression and start motors without having to lug an entire trans around the shop floor.
i believe the 4 speeds are magnesium cased also?
On the GC 4-speeds (I like the to keep the GP and GL tranys), flanges make nice axle caps for stored axles.