Wheel Offset??

I am building out a 1974 Duster and need to order wheels and tires. Wanting to go with Cragar SS Style wheels and want to Run 275/60 R15 Mickey Thompson ET streets on the back.

Running 8 3/4 A Body Rear End. The car has Ladder Bar rear suspension and 3" Offset hangers for the rear leaf springs so lots of room inside the wheels wells. I was hoping to tuck the wheels fully inside the wheel wells so I could bring the Back end of the car way down.

I was looking at 15 x 8 wheels for the rear and 15 x 7 for the front. It seems all the stock wheels I am finding only come in a 4.5" BackSpace.

Anyone running 275/60 R15's on a 15 x 8 Rim with 4.5" backspace? If so, how well does it clear the federwells?

Also what size tire would you run on the front for a 15 x 7 Wheels with 4:5 Offset? Want to fill up the wheel well as much as possible without scrubbing anywhere.