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You won'tl have any problems with the 33's i have a 76 Hiboy and is running 35's so 33's will fit just fine on your 79 F250 even with the header's. But just to be on the safe side you might wanna have the rims 2 ot 4 inch offset that will give an some more clearance.
I have had 35's on my 78 F150 4X4 with no lift and no wheel offset so I can't imagine that it would be a problem on a 250.
I would NOT recommend adding any offset in the wheel. This puts a LOT of strain on the wheel bearings/spindles in the front and axles and wheel bearings in the rear. It is a bad practice that should be used ONLY if absolutely necessary, IMHO.
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