Yes, that's the absolute wrong way to call wheel sizes out...
Here are some numbers for you so you don't have to go look them up.
You'll never fit anything over a 305 on the rear with a stock body. Ever. Not gonna happen so don't even entertain the idea. Kevin Doe widened some 18x10" rpf1's to 10.75" wide (an honest 10.75" wheel, which actually measures 11.75" including the bead locks in case you didn't know) and was able to fit a 305. He had to use a completely offset trailing arm, roll his fenders absolutely flat, and pound the inner tub a little. It still kisses the outer fender and inner fender well. That being said, here are some more reasonable numbers for you.
18x10 +38 rear is the perfect offset for a 285 in the rear. It will require rolling your fenders but that's easy. You could get a 295 with a +43 offset and an offset trailing arm if you wanted. The front is all about whether or not you like rubbing and your fender liners. 18x9 +40 will fit a 255 or 265 and will probably require flattening the liner bolt tabs and using sip ties in place of the bolts. You can also fit an 18x10 +50 in the front to allow for 285's if you wanted.