Back space help please!!

I prefer offset. that tells you where the wheel mount face is. a 8" rim is 9" wide overall, so 4.5" backspace is the same as 0 offset (the wheel mount face is perfectly centered in the wheel, 4.5" of the wheel outboard, 4.5" of the wheel inboard). a 7" rim is 8" wide overall, so 4" backspace puts the wheel mount face perfectly centered, in the same position as the 8" wheel with 4.5" backspace. a 7" wheel with 3.5" backspace has .5" of negative offset, so the wheel mount face is inboard .5", so the outer edge of the tire will be pushed out .5" compared to what you have now.

Typical muscle car era mopar rims have ~1/4" positive offset (so a typical 7" mopar rim has 4.25" backspace).