You might be able to get away with only buying that front portion of the inner roof panel if that is the only portion that needs to be replaced. But like you mentioned you will have to deal with the issue if that smaller panel has the extra reinforcing pieces on the back side, or if it doesn't you will need to try and save those on the old panels and spot weld them onto the new. I would call Classic Parts and ask if the new panel has those pieces, it might.
Your jig looks really good for keeping the windshield opening correct. You might want to throw in some cross bracing on the inside of the cab from corner to corner and in between the door openings as well. This will help keep the cab square once the roof is off. I've seen people go without this bracing when they are chopping the cab, but I feel like that just leaves the door open for things getting squirlly and all kind of fitment issues down the road and more work, my opinion.
Question1: I assume you are talking about this panel shown in the picture? So this section actually comes as a part of the new outer roof skin. I was worried about it too until I saw it on the new one. There is another member on here MP&C who has shown how to make this with a metal shrinker and stretcher, and I want to say that I think I've seen this piece new on eBay from some sellers. On the instillation part you just clamp the inner and outer panels together with a sh**load of vise grips. Before I clamped them together I used one of those pneumatic metal hole punchers from harbor freight to punch spot weld holes about every 1.5 inches or so around the outer roof skin. Use the factory assembly manual on a guide on how many and where you need the new spot welds.
Question2: I did not braze the new joints together. Wish I did and knew how too, but I just welded the seams and then ground smooth with a roloc disc. Eventually before I go on to painting the cab I will use a seam sealer from 3M in the drip rails to make sure nothing leaks.
