Does the axle in your picture measure 63.5" from WMS to WMS?
That's what I would have said, but I just now braved the cold to measure it and discovered its 67 inches. The axle has been under there since Thanksgiving of '08 so its been a while since I last measured it.
There are 4-3/4 inches between the backing plates and the springs and closer to 8 inches between the backing plates and the frame.
Subtracting half of 3-1/2 inches from each side I believe you'll have plenty of room.
It may be that if I tried to put a standard box on this truck I'd have a tire interference problem. Since I've not been able to find one its not an issue.
