I didn't remember you have your frame flipped over. Lots of work to do that, good job.
A lot of people use 5052 H32 aluminum sheet .025 thick. It is lower cost, easy to work with, and adequate for the job it has to do. The belly pan is a simple cover, and carries very little load. This material typically comes to you in 48" width rolled up to the length you need. I ordered mine on line from Air Parts in Kansas City. Good people to deal with.
Riveting the belly pan on is straight forward. I decided to run the 48" dimension side to side since many of the cross members are on 24" centers. I provided about 1" overlap at the seams. Some seams were on cross members, and some seams were nothing more than a "stitch line" of rivets holding the two sheets together. I cut the 48" wide aluminum to the length I needed, usually in the 80" range, although narrower at the axles. All of the belly pan was riveted to the cross members and frame rails and outriggers.
I used mostly large flange 5/32 "pop" rivets cinched with my air riveter. If you want to strengthen your hand grip, you can squeeze them tight with a hand riveter. There are lots of 5/32 diameter holes you will need to drill. Most of my rivets are on 9" spacing. DO NOT worry about making your belly pan water tight. You want the rain water and road splash water to drain out. No water sealing needed. You don't want any holes bigger than 1/4 of an inch in an effort to keep mice out, especially propane line penetrations and the like.
I did the son's 69 Globetrotter belly pan this last winter. It is a much harder job on your back under the trailer. It will be much easier for you.
Gotta get your tanks, drain plumbing, any wiring, and floor insulation done before attaching the belly pan aluminum. I put my axles on after the belly pan was up.
Hope this helps...