Paul,
I have a 3600, so I have a slanted opinion.
Yes the 3600's have 4.56 rearends, and are "slow", but axle swaps are fairly simple. 3100 guys have the same problems. a lot of them end up looking for 3.55 gears, or something faster.
The original 3100/3600 drivetrains would have been similar. a 216 or 235 inline 6, and either a 3 or 4 speed transmission (the 4 speed just gave you a low 1st gear to start out with heavy loads) neither had OD.
The 3100/3600 use the same hood/front fenders/cab so there is no difference there.
Now you have to look at what has been done to each individual truck over the last 70 years. Has the motor be changed/upgraded? Has someone added an overdrive transmission? Has someone changed the rear axle or gears? What other mods have been done? This varies a lot truck to truck, anything from the purist restoration, to the low rider on air with a big v-8, and everything in between.
It may be difficult, but think you have to decide what you want the truck to be, and make a personal decision. There is tons of information on the site from purist restoration, to hot rod, to ratrod, to restomod. engine swaps, axle swaps, suspension changes....The list goes on forever.
Good luck!
Jon
