Interesting that you say they hit before they got over the studs; for me they only get close when they are over the studs. Until that point the drums aren't close to the drop-center of the wheel where they would interfere. Mess around with it a little and see what you come up with.
Some of my crap:
1963 C-30 flatbed dually 292/SM420
1965 C-10 LWB fleet 250/column shift 3 speed
1965 C-30 9' stepside 250/SM420/4.10 gears (my DD)
Website I made for my crap:
http://www.66submarine.com 
