Wheel Math help

the new rear end that i am using is shorter than the OEM one. here is the deal, i borrowed some wheels with a 42" offset and end to end it measures 52" inces with the wheels and tires on it. The bed clearance is 56" till it hits sheet metal.

If i change the offset do get the wheel close to the bed so it doesnt sit in??

i want to extend it to have about a 1/4" from the bed.