The boom issue with box trailers hooked to the 5th wheel on the 819 does not get better if the boom is over the cab because of the rear overhang of the crane. If you mounted the winch between the crane and the cab, that would take a lot of room to get the fleet angle right so the cable will lay on the drum right when taken in. I believe that if you mount the 5th wheel where the winch is, you will have a very tail heavy unit with a LOADED flatbed attached, things will be just like the lite front end when lift towing, the 5th wheel needs to be where it is on the 819 in order not to have a light hard to steer front end.
What I would do is stretch the frame back BEHIND the rear winch, LEAVE the winch because the no rear winch issue is the major drawback of the 819 though I will be correcting that on my 819, move the rears back to the new REINFORCED stretched frame, mount the 5th wheel there, centered just in front of the trunion. One thing you need to remember is that your crane reach even stretched out over the rear will be short, with the only 18' of boom flat out, about the only thing that can be reached would be right behind the 5th wheel, the 819 on the other hand has 26' of reach, without the second section out, the to the rear work area is very limited.
SO my suggestion is to find a 819, build on that platform to what you want, there is a LOT of reasons for this suggestion,
1 -- A triple frame under the crane.
2 -- Heaver stock tires/wheels (1200x20 and even these are to lite for the front)
3 -- Heaver front hubs
4 -- longer boom (26')
5 -- Rear outrigger on the very rear as compared to a stretched wrecker.
6 -- The rear outrigger pads come off for trailer pulling.
7 -- The ramps are not just there but ARE part of the triple frame .
8 -- There is plenty of room for a right hand fuel tank AND under bed boxes on both sides.
9 -- EVERYTHING that you want to do with you wrecker is ALREADY there, so instead of spending your time and money to BUILD a 819 CLONE from your wrecker them mod the build, just mod the 819 to how you want it, just modding you wrecker will take a LONGGGGGGGGGGG time, then MODDING the MODDED wrecker will take a long time, WILL YOU LIVE LONG ENOUGH TO SEE THE FINISHED PROJECT. WHY reinvent the wheel the wheel, just polish it.
There is no point engineering the 819 clone when it is already done for you, the drive line is already done for you, the high points of stress are already delt with for you, AND I believe that it will be CHEAPER for you to START with a 819 platform AND I also believe you will be HAPPIER in the long run.
REMEMBER once you use a m819 there is NOTHING to go back to.