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1948 Chevrolet 1/2 Ton Truck (w/1951 chassis): I am replacing the gaskets on the Rear Differential Cover & Driveshaft junction. Per factory specs, one gasket is for the Rear Cover (a thick gasket) and the other for the front driveshaft to axle junction (a paper thin gasket)

I'm thinking about using a Thicker gasket (like the one for the rear cover) on the front for the Driveshaft to Axle union.

Question: Is that a bad idea, anyone else ever done that before?

By using a thicker gasket on the front you will put the axles out of alignment in the wheel bearings.

No reason to.
Use a flat file to remove any nicks and burs on the mating surfaces. I like to use Permatex #2 sparingly on paper gaskets, you might have your own preferred sealant, hopefully not silicone, in any case don't go overboard.

As mentioned the thin gasket is ideal for alignment with axle shafts as designed, no gasket or a thick one would be a bad idea.

Thanks Pre 68 Dave & Grigg, I will heed this advice and use the paper thin gasket w/some permatex (to help it stick). I was having a hard time getting the gasket to stay in place as I brought the driveshaft back up to bolt to the differential hence my thought on using the thick gasket there (it doesn't move so much), but this permatex on the paper thin one sounds like the ticket...and it will keep everything in alignment.

One more question: When I put back all the gears inside the rear differential, should I coat them liberally with some Mobil 1 Heavy Duty grease? I realize the unit will be filled with Gear Oil when I'm done, but just wondering if I should also, initially use some grease on the gears?

I have rebuild many diff's and have never done anything but fill with 90w.

Permatex makes a gear oil rated liquid gasket material that I've had good luck with for the rear cover. It applies like silicone, but makes a tougher gasket that doesn't get degraded by the oil. The front cover paper gasket can be held on per Griggs suggestion, I've also used the gasket maker that Kubota sells for many applications on its tractors. I'm sure there are other equivalents out there. It is very thin and can be brushed on. It sets up as a stiff rubbery gasket that makes an excellent seal but is paper thin. It runs and makes a mess if put on too thick.
As far as greasing the gears, if your really concerned, before you drive off jack up the rear and turn it over by hand a few revolutions to coat the gears with some oil without loading them. But unless your doing your rebuild on the starting line of a drag strip, easing out of your workspace to some normal driving will provide plenty of lubrication within a few feet of travel for normal break in.

Many thanks all!! This is very helpful information. My first re-build...

Grigg, I used the #2 Permatex with paper gasket and it worked great, held it in place so I could raise and mount the driveshaft without it warping on me. Thanks again, plus 1 more question - one of the nuts on the rear diff housing (driveshaft side) was threaded or worn out. I found a nice 3/8-24 nut with a locking washer to replace it, but that got me thinking (the new nut is stainless steel) why not do all of them now? The original nuts are spliced on the outside edges on each flat side and tapered (probably for pressure lock) but do you think it would be OK if I just used regular nuts in conjunction with the Locking Washers?

I don't think the nuts are a big deal, use what you please. Some sort of lock washer or locking nut would be good.

Think a nylon lock nut would be a good choice for the worn stock nut.