Back Spacing, which way?

To simplify, it sounds like you want to move the tire in about 1/2" to center the tire in the space available, so your new backspacing should be around 4 3/4" with the 8" wheel. That is equal to a positive "offset" of 1/2" with the 8" wheel.

Don't forget, for measuring backspacing the 8" wheel is actually 9" lip-to-lip. Nearly every wheel made is 1" wider lip-to-lip than the wheel's bead width (8" here).