Roadrunner back spacing

[crm440]

Looking to put the tallest and widest tires that will tuck in my wheel wells on my 73 roadrunner. Any help on backspacing and tire size would be great.
I have a 1970 8 3/4 rear in it.

 

[74sundance]

I know a guy that has:
Wheels 5x4.5 8.5x20 -14mm
Front 255/35/r20
Rear 275/45/r20
On his Roadrunner plus he has lowered the car.