Hi Chip,
I don't know the gauge of the Thiokol's aluminum skin, or which alloy they used. In my experience while sheet metal is referred to and sold by "gauge", aluminum is sold by the decimal size. It is also important to specify the alloy of the aluminum you want to purchase. Different alloys have different properties (and of course different price points)
On my Tuckers, (1979 and 1980 vintage) they used aluminum for the hoods, roofs, door skins and floors. Tucker used .063 aluminum in 5052 alloy. My snowcat buddy and I have used both .063 and .080 aluminum, but we've stuck with the same, 5052 alloy.
My snowcat buddy picked up a Thiokol 1201C this spring, and my recollection from looking at it is all the rivets are the same size. My guess is they used 3/16 rivets, but that's a guess. The Eastwood kit seems divided into two rivet sizes, 1/8" and 3/16". Before purchasing that, I'd suggest you look at Aircraft Spruce and Specialty Company, and see what rivet components and tools they offer. I think rivets can be sourced from McMaster Carr as well.
After completing some Tucker projects, we are also going to modify the cab on the 1201C, I suspect in a similar fashion to what you are doing, but with a couple of differences, which you might consider.
If you look at the early Thiokol 1200 vs later LMC and DMC 1200 series machines you'll note the later models have taller doors. After climbing into Scott's machine one time, we agreed it required some contortions, and the taller doors would make that a lot easier.