You could check with a local RV dealer (or two) on recommendation on who could do the frame modification. I'm sure they have had to deal with this issue before. Also I think any tractor trailer repair garage would point you in the right direction. There are garages that specialize in tractor trailer suspension/spring repairs and they probably would have the expertise in this endeavor as well. I would think that you are going to need a minimum of 6" clearance between the top of the bed and the bottom of the fiver. That is what you need to find out from the shop; with a frame modification, will that give the clearance needed and what will be the overall height of the fiver once that is done ? I gotta think that there is a way to do this as RV'ers are towing with new Dodges every day. Just a matter of locating the right shop.