For rules you can Google search for something like "SCCA GCR", GCR is general competition rules I think. Roll bar and roll cage tubing is generally thicker because it should withstand abuse and avoid being crushed which would weaken it.
My car is 1.375" x 0.083", which is the lightest allowed tubing and should be about where you can go. I think, but don't really remember, that 1.5" of that thickness would be OK too. On Tom's Car9 he uses a single tube to the floor without the extra small bends in the legs. That's because it might be hard to get those bends right and there is also a limit to the total number of degrees of bend in the bar. I used the kink you see, but it isn't one tube because I bought mine from Online Metals with an 8" shipping limit.
I don't have a strong opinion here on this. Not much is wasted if you choose to buy thicker material for your roll bar, you got practice and all the straight material can go into diagonals and welding practice.
I don't think you need to raise the rails for that motor. For one thing the height of the rails and the height of the bonnet are not the same thing and you can just add a bit to the bonnet and instrument panel when the time comes. I also had a Duratec in the models so we can check that again, it's probably about the same height.
The height of the upper rails was worked out with Jack McCornack to fit his Lalo body also. I think part of the choice had to do with being comfortable for the drivers elbow, something you can try on your build table. On the other hand I think Tom found the height to be at a lower limit of what he could find coilovers to fit so raising an inch doesn't hurt.
You doing a nice job, the front bulkhead takes a little effort to figure out a nice way to join the parts up.
We have a new track in the area at Palmer, MA. It looks beautiful and is closer to you. My favorite track was the old Bryar in Loudan, NH. Perfect for these cars with short straights and lots of turns. Plenty of drivers with really fast cars would realize they didn't actually know how to drive when they got there.