Agreed GMAN...this is not your typical Combination. A full time flatty is going to use a lighter Tractor. Most likely when you see a 770 hooked to a flatbed...it's not his usual run.
Depends on your accounts. I knew an oldtimer who hauled this way all the time. Guess his loads always were the same and permitted this with no trouble.
If I remember my 770...I cringed at anything over 42k. There were some loads I ran very light on fuel.