18" wheels

A 255 front would need a 8.5-10" wide rim
A 275 rear would need a 9-11" wide rim

I say go was wide as you can, large rim diameters, low profile tires look anemic when they are skinny. Measure it up when you have it together. Or if you haven't changed sheet metal around 72BluNBlu can probably get you in the ballpark.