Back spacing help

[MN67C20]

Hey Everyone,
Sorry to bug everyone with the same old question but I am at a loss and need to figure this out. I am running a Eaton Ho52 rear end with a 65 inch width. I have converted the truck to a short step with the stock rear axle with a disk conversion. I also did a full front swap to a 79 c30 front end and that is 66.5 wide. I am installing air bags and want to tuck the tires when aired out. I want to run the same width tires front and rear on 18's but have no idea what backspacing I can fit. I would like to run 8 or 9 inch rims but am open to ideas. The measurements off the wms in the back is 4.25 to the lip and 7.50 the box side. If you have any ideas or suggestions please let me know.
Thanks