My car is a 340 Formla S. Numbers motor (under bench now) and original fender tag....all the mods you mention are bolt on anyways.
For the backspacing, just ask Dr Diff if the brakes move the rim mounting surface inward or outward and how much from a stock 73-76 large bolt disk setup. Have you asked him?
For a 73-76 large bolt disk swap, you can run 5.25” backspace most of the time a little more.
245/45/17 17x8 with 5.25” backspace will work up front.
In rear with large bolt axle swap and factory drums: 245/45/17 17x8 with 4.75” backspacing will work. Or you can run 5.25” like front with 1/2” billet spacer.
