Caliper Clearance + Tire Question

I cannot answer you question.
But I have this to offer;
In my experience;
The tallest tire that I could install in the back of my 68 Barracuda, was a 28" ; AND I had to move the axle back a tad to do it.
That was a 275/60-15.
The 275/65-15 that you mention, maths to 29 and change.
The 15 x8"/ 4.5bs on the stock 8.75 rear end will barely fit into the tubs, wanting to rub on the outer lips. With a lowered ride height (mine looks like yours) the tires want to hit the tubs on the outboard sides were the steps are.
In my experience;
on the street, with street tires, any front discs, and the stock width rear end;
you will never be able to use more brake on the back than the 10x2 inch drums.
I have the same type of wheels as you do. But 14 x7.5 on the front and 15x10 on the rear. Tires are 235/70-14 up front and 295/50-15 on the rear on a narrowed rear end with offset spring-hangers.
On the Front, My Barracuda has the KH 4-piston DB set-up. I run zero proportioning and 15/16 rear wheel cylinders. I also run; a 15/16 M/C and a booster from a 73 Swinger, IIRC.
The car stops exceptionally well on the street, IMO, having saved my azz several times.....

Good luck on your quest for answers.


BTW
if yur only gonna run 215s up front (about 7inches of rubber), then you really don't need much brake to skid those. Discs on the back would have to be pretty heavily proportioned to NOT skid first......... which sorta defeats the whole purpose of having discs back there in the first place.
About the widest 15" tire you can fit on the front is a 245, and that is a chitload of work to make happen, on the factory DB spindles and small UBJs. Not to mention custom BS wheels.
For me, after my 245s wore out, I went back to 235s, and learned to steer with the gas pedal. The 235s provide adequate stopping with the 295s in the back running zero proportioning. It's like I imagine tossing the parachute might be like......