Wheel Size Question.

My 69 Barracuda takes a 4.5" backspace with 0 offset on a stock 8 3/4 SBP rear and stock suspension. You safely have 5.5 inches from the front of the drum to the leaf spring and an almost identical distance from the front of the drum to the wheel well lip. So your 4 1/8" (4.125) backspace with a 3 mm+ offset would equal 4 1/4" total backspace, or very close. You're definitely safe with those rims. Now, don't sell yourself short on rims due to the 5x4 bolt pattern. If you check American Racing Wheels (for example) they offer certain wheels as "custom". Those wheels can be done with the SBP and you set your backspacing and offset. AND.....you get a 17" or larger wheel which offers a lot more tire size options. I am waiting to get an order in but Covid has slowed everything down.