If it's only the bolts in the way of the tailgate, maybe you could remove one at a time, countersink the hole and use countersunk screws?
Or, since you haven't painted yet.......modify the front of the bed to fit it in?
I don't think I can countersink the hole since the metal is very thin but I have not mounted it yet so maybe after it is mounted the bolt will clear the spot in the tailgate. I could modify the bed though. Also have to see if the tailgate allows for good air flow to the burner or not.