8" wheel with 3 3/4" backspace on 72 Scamp fitment

Do you still have the 5x4 pattern? Most BBP conversion axles widen the rear track width.

If you remove your rear wheel, and the car at ride, with a square laid flat against the drum face do you have 5.25" of gap between the inner fender and drum face? Now you will want more than 5.25 for tire room, more like 5.75 to 6"