R&P swap isn't impossible, especially since it's 2WD, only 1 set to do. You'll need a caliper micrometer, dial indicator, hydraulic press or access to one and a bearing cup, and a good, high-torque air impact in the neighborhood of 450 ft/lbs or better. You'll need a thick gear set to run 3.73s on a 3-series carrier. Avoid the shim plate R&P sets. You'll also need a bearing and install kit.
When you pull the carrier, if the shims are between the carrier bearings and the axle housing you need to catch them and note their thickness and orientation. It will come in handy to try to put the same amount of shim back when you re-assemble and get you in the ballpark. Some 10 and 12 bolts have the shims under the bearing between the bearing and carrier. In that case you'll have to pull the bearings and mic the shims. You'll want to pull the bearings and replace them either way. No sense in running 40 year old bearings on a new gear set.
The pinion bearing races can be removed from the housing with a drift and hammer. You'll need to press the bearings off the old pinion so you can mic those too. Some people dont. They'll throw 0.010" in there and see what they got which is fine. It is likely it will have to be pressed off again and more shim added or some removed either method you try. The pinion bearings have to have a measure of preload on them when the pinion nut is tightened. For setup you don't have to be exact but you don't want it to be loose. Usual preload is about 24 inch-pounds so it's barely discernible as drag but it's too low to get right with a ft/lb torque wrench. If you can spin the pinion easy enough that when you let go it stops it's just about right. It will run in a little anyway. Most of your ring gear bolts are going to be left hand thread so don't be surprised if you have to set your impact to "righty loosey". Also they're assembled with thread locker so they will be tough to get off.
Once it comes time to place the carrier, you'll want to put your shims in after the fact between the race and the axle housing. You may need a pry bar to get them in, it should be a pretty tight fit. A ratchet strap wrapped around the axle housing will help hold the carrier in while you put your shims in and bearing caps on. You'll torque your caps on and then run a mesh pattern to see how it looks. Don't worry about backlash right away unless it sounds like it has way too much. Moving carrier shims around will tighten that up. There are a lot of google images with what to look for as far as mesh pattern is concerned. You move shims from one side to the other till you tighten up the pattern but not too deep into the root of the gear. If it's too deep the pinion needs shim removed. Too shallow and the pinion needs a little more shim. All this also affects the pattern in relation to the heel and toe of the ring gear.
Once you get a good looking drive and coast pattern check your backlash. You're looking for typically 0.005" - 0.008" for a new gear set. A lot of times just moving shims left or right of the carrier can adjust your backlash into spec but it may mess with your mesh pattern which may make you have to adjust your pinion depth. If you're not the kind of person willing to handle a 30 pound chunk of iron and steel a dozen times to get it right, don't bother even starting. If you are and you've got it this far along, there's a crush collar that goes between the pinion bearings to set preload. If you overtighten the crush sleeve you can stretch it back out with a hammer and the horn on an anvil or back of a vise. Some people opt for the crush sleeve eliminator kit which is just a stack of washers and some shims which takes a lot of trial and error to get the preload right.
Once you're finally ready to tighten the pinion nut, use plenty of red thread locker, and use a chisel and hammer to to "ding" the nut to make it hold stronger. It takes a lot of impact to run down the pinion nut. You can put a pipe wrench on the pinion yoke and let it turn over against the ground while you're tightening. You do not want to use the impact on the pinion nut with the carrier and ring gear in place. It can damage the gear teeth.
So there ya go. You may want to just look around for a 3.73 or even 4.10 rear end to see what's out there. Use a gear ratio calculator and fine tune your final drive RPMs with tire size.
