If I had a 20x8 inch wheel with an 1.25" wheel adapter, would the wheel stick out of the fender? The wheel also has a +40mm offset.

 

TTT

 

I'm not an expert but I think it would. The offset for fbody wheels is 40 to 65 mm I think so when you add the adapter, that adds a lot of extra space.

Personally I wouldn't go skinnier than a 9 inch wide rim on an fbody, especially on a 20 inch rim, because it doesn't look right.