sorry guys, you are both wrong. you hafta consider the application, and the width of the stock wheels before using BS as a measuring tool. the factory rims which are 4.5" BS are only 7" wide. he is lookin at 9" wide wheels. to keep 9" wheels centered in the wheel well similar to stock he would need 5.5" BS. that would put the rim lips 1" in and 1" out from stock respectively. now with 5.875" BS his new wheel lips would be 1.375" in and .625" out from stock.
on a lowered truck like his will be, they will fit in the wheel openings just fine, and still have clearance on the fenders.
the only issue you might have would be the wheel rubbin the lower shock mount at full lock. especially with drop beams as some of them dont have steering stops built in at all. there are ways around this issue as well. many many 18" aftermarket mustang rims are 24mm offset, and alot of ranger guys run these on thier trucks. get under your truck and take some good measurements of all the places a wheel might hit, especially while turning. check out alot of other guys wheel combos and make sure you get all their info including if they used any spacers to clear the hubs or what have you.