This is my 85 Crew Cab Dually on a 19.5' wrecker bed. Note the back tires are barely on. You can cheat a bit though with your trailer because you can overhang the front a wee bit. I will drag out the tape measure tomorrow if I get a chance and get you a measurement.
I would also suggest that you check with a couple of towing companies in your area, explain to them what you need to do, that it is not an emergency tow, that it could be done when they are slow and they might cut you a break, I do that sort of thing all the time.
Just consider this, paying a couple of hundred bucks to get a pro to move your truck for you will be much less expensive and less of a headache than trying to do it yourself if things go badly.
I do believe Yukon Jack can tell you all about this.