You don't need to touch the pinion gear depth, shims, even if you have to change the pinion yoke You should, and need to, check the backlash in 3 or 4 places opposite of each other on the ring gear before removing the ring gear on the center section. It needs to be put back in with the same backlash + or - .002 either way The Mopar factory service manuals from 1966 to 1971 cover every aspect of working on and setting up properly the Dana 60 as well as all the other rear ends offered in the cars those years
You should buy one those if you want you want to set the new posi up yourself. The posi unit, center section carriers, needs to have between .012 and .015 positive preload. You should buy a good dial indicator(one inch travel measuring in .0010 increments) with a magnetic base to check the back lash and carrier preload I don't use a case spreader either, I use two pry bars to carefully pry the center section out and soft dead blow hammer to carefully put the center section back in I remove the carrier bearings off one of the old center section I had and used my die grinder with a sanding roll to hone the inner race so they would slip on and off easily on the new posi unit to help check and set the pinion bearing preload (Same thing on the rear pinion bearing ) Once you have the prelaod and back lash where you want it then remove them and install the new bearings on the shim packs you have on each side, don't change the shims after you have them set up Make sure and recheck both the back lash and prelaod, the carrier should not slide in easily after the new bearings are pressed on
As far as shims Dr Diff offers kits for them, I bought several kits to help avoiding the shipping for the next two I will do The oldeder Dana 60 use shims to set up every aspect on them, pinion depth, pinion and carrier bearing preload and backlash. I have two junkyard truck and van 3/4 ton Dana 60 in the shop to use on future projects
As far as how much to narrow your rear end for your car are you happy with the way the current wheels and tires fit? Are you going to reuse them or change them? If changing them you will need to use them under the car with the rear end out to measure how wide the new rear end needs to be from axle flange to axle flange by measuring to the inner mating surface on the wheels Take you time in positioning them in the wheels well to get them perfect Mopar used a offset on the passenger side on all thier production car rear ends to get the pinion gear in the center of the driveshaft tunnel, I think it is between 1 1/2 and 1 3/4 inch shorter on the passenger side axle. Maybe someone on here can post a link to a past post with the exact measurments for the different year cars with the stock Dana 60s measurements
Don't forget to buy a new set of leaf spring mounting pads to use on the your Dana 60 housing, you may need to make them fit the larger axle tubes if yours are bigger O.D. than the stock 3.0 housing where Mine are 3 1/8 and I have seen some out of a 1 ton that had 3 1/4 inch O.D. tubes on them They all will work good
Good luck, welcome to the hot rod culture of doing it yourself