I'm about to purchase new gears myself. I plan on keeping the low gear ratio since my little 170 slant 6 won't ever haul ***, i'd rather it save me some money.
here are a couple of sites where i found some items. I can't seem to find just a shim kit though.
Edit2: It also looks like summit racing has a few parts for a chrysler 7.25 rear end. They had a couple ring and pinion, i think 3.55 and 3.90. the sites above have much wider selection. Looks like they just have pinion shims, if you were looking for a rebuild kit.