Not sure if this helps or not, and everyone's car's like these are going to be a little different on the outside, but should be the same on the inside. BTW-love your car, looks great!
On my RSR racecar '71 chassis, alum SC/Carrera trailing arms, which is lowered significantly, but runs a fairly tall rear hoosier tire at 25.5" (front is 24.5"), I have found the wheels that setup the best for me, clearance wise/tire size wise are 8.5" fronts and 10" rears. These wheels are 2pc made up using the original 6" fuchs centers with a BBS bolt on outer, roughly a 5" backspace (Bill V. knows this by heart, these are 16" wheels), but you may be running the same 245/275 widths in a 15" tire I would guess? Or bigger? I also have a set of 3pc HRE fuchs in the same widths/setup with 5" backspace f/r 8.5F/10R. I don't know how to calculate ET, unless he is phoning home. Both provide ample clearance to inner oil lines and trailing arm bolts.
In the rear I run a 1/4" spacer, if I run a 1/2" spacer the tire will rub up inside the rear flare, but only when I am 3-wheeling...so you really have to push to get that to happen. Probably not going to happen in street conditions. I can squeeze a custom 9" in front, same backspace, but it is really tight on the outer flare lip. The outer edge of my tires sit much farther inboard than your pictures show.
For cars at such a light weight like ours (2200-2300lbs), I am not sure how much the delta is between a 275 tire and a bigger tire in the rear, but I can run faster than similar RSR type cars like ours with bigger motors and more horsepower and the same weight that are running a bigger tire package. Like, multiple seconds a lap faster and these drivers are not slouches. Maybe its a heat thing, easier to get the tires up to temp?
A couple photos for reference
