Jamell, it appears you have a pretty nice 3600. You should find a metal tag between the drivers door hinges like the one below. If still there it'll likely say 3600 however since your truck was represented as a 3100 the tag may well be missing.
On the brake drums, I'm confused, why do you need new drums (which by the way aren't available for the front)? You stated you had drums that showed no wear, I lack the words to say how good of a thing that is. In your shoes on the rear brake shoes, I'd wash em in a good solvent and or clean with carb cleaner then lightly sand with 80 or 100 grit sand paper. While sanding I'd ware a respirator just in case (unlikely) asbestos was present. Then I'd pull the wheel cylinders apart, do a light hone, install new seals and then do a complete brake fluid flush and fill with new fluid, adjust the brakes and take a test drive.
The fact that you seem to have 4.56 gears is another vote for stock 3600. Lots of info on this site about swapping the pumpkin out for a set of stock 4.10 gears from a late 60s very early 70s C20 Chevy/GMC truck. That along with going to a S10 T5 (overdrive 5th gear transmission) makes for a very nice highway drive-able truck.
