3" inboard wheel backspacing

Going from a 7.5 to a 6.5 moves the outer lip away from your mounting face by 1". That 6.5" would work if you were to use a 8" wide rim.

Best thing to do is use a plumb bob on the three points mentioned in the BBD write up, inner, outer and mounting face. Get your wheel centered in the well using those three points. Don't be surprised to find one well is not the same width as the other... BTDT My mini tubbed car has 15.25 width on one side and 15 on the other. Yeah they built it real square.