I agree with Andy on the gooseneck to fifth wheel conversion on the trailer. My living quarters race car type trailer was a gooseneck when I bought it, and I got REAL tired of how far I had to crank up the landing gear to clear the ball (I hook and unhook a lot). I was previously spoiled by my last trailer which was a fifth wheel, and nothing beats just spinning the landing gear down with 2 fingers til it touches the ground, dropping some air out of the air bags on the truck, and driving off with the trailer still sitting level.
So, I got one of those slide-in fifth wheel conversion adapters for the trailer. No problem so far. BUT the pin height is lower than a similar trailer that was set up for a fifth wheel in the first place, even adjusted up as far as it can go. I have an adjustable Reese fifth wheel hitch in the truck that is lowered down as far as it would go, and the truck was already lowered by 4", and it all just barely worked with the trailer level. The same setup with a stock truck would not work. For me, it all worked out great, gooseneck hitches area pain in the ass.
It is going to depend on your trailer hitch vs. the truck if you can make it work with the bolt on adapter, but it sounds like you already measured and that won't work. Any chance you can remove the fifth wheel, make a new mount for that in a lower position on the truck that will work with your trailer and an adapter on the trailer? If you a have to fab a whole new mount for the gooseneck hitch on the truck anyway the labor would be about the same, and the trailer adapter is only about $150 for a good one. If you can make that work, WAY nicer to hook and unhook. As another option you could have the trailer converted to a fifth wheel (new hitch fabbed and welded on) which is short enough to work with your truck as is. Again, the labor may not be any worse than having the truck changed around.
I just bought a Trailer Saver air ride 5th wheel hitch for my Topkick, which will be about the same setup as your Freightliner. I already know the hitch will have to sit down between the frame rails somewhat as opposed to up on top of the rails, perhaps yours could work out the same.
Just brainstorming. Good luck.