Most likely, you have 4.56 gears, if not those, it will be 5.13, as those were the only options for 3/4 ton. either way, you do not want lower gears, as that will make the situation worse. You want a higher gear. About the only direct bolt in option would be to fins a '68-'72 3/4 on 3rd member with the 4.10 ration, mostly found in trucks equipped with a 350 and automatic (some 396/402 trucks got that gear also). Beyond that, you'll be looking at swapping a later model rearend under it.
As far overhaul parts for the 235, you should be able to get them from the local parts store. Federal-Mogul still makes all the gaskets, bearings, rings/pistons for them. If you can't get them locally, try Patricks or maybe Kanter.
