Re: rear end gear ratio change
One can easily get ring gear/pinion gear kits as well as the bearing kits at the truck parts house. Yours is a common axle. It would probably be easier just to buy a good used highway geared rear axle out of a normal box truck with the same brakes, pull the cover, and check the teeth and install the whole rear axle, if you don't want to change out the ring and pinion yourself. Your ring gear probably takes rivets to put on, and that requires a press. Bolts will work as well.
For the cost of the parts, i would probably just change out the rear axle for a newer good one. fwiw
Jesus Christ... Conversion in progress.