Sorry For Another Wheel Question
 

I've searched and searched but can't find the info needed. I have a 1972 Cheyenne Super Short bed. Why is it everyone runs 20x8.5 boss 338s up front? By all my measurements im thinking i can clear 20x10s up front with a 4.5/6 drop. Granted I could be an idiot with the tape measure and search feature. Thanks for your help in advance