[67ChevyRedneck]

You're not going to notice much difference at all going from a 3.73 to 3.55, maybe a few hundred rpm, which is kinda "nothing"... and what came with a 3.55? That's not a common ratio I remember being in these trucks...

The vast majority of manual trucks came with 3.73's. The vast majority of automatics came with 3.08's. The cheapest, quickest thing for you to do is to find a 3.08 open rear (they're not as "popular" as the 3.73 or posi), look around the pumpkin for signs of heavy leaking (leaks around the rear cover are OK, a new gasket can fix that, but a leak at the yoke requires a lot more work) pop the cover, inspect it for wear, pull the drums and look to see if the axle seals are leaking, and if all looks good, swapping the rear axle can be done in just a few hours and maybe add a day if you have to redo all of the brakes. Also remember, 67-70 rears will be 6 lug and 71/72 will be 5 lug. It looks like you have a 71, so you'll need a 71/72 5 lug which is more popular because of the 5 lug pattern... realistically, expect to pay $150-250 for a good useable rear. I picked up a 3.73 5 lug rear last year that with a little clean up could be run under my truck immediately for $250. These trucks have really well built/tough rear axles. I'm pretty sure mine has easily a couple hundred thousand on it and it still has the original axle seals, the pinion doesn't leak, and when I replaced the oil a few years ago the gears looked new inside still, so a good used rear should be easy to find.

Your truck will be fine with a 3.08... hundreds of thousands of them came with the 3.08. AND most people that have swapped to OD have 3.73 gears... which honestly, by the time you do the math... having OD with a 3.73 is almost equivalent to having a 3.08 rear and no overdrive...