Sorry Mike, I live in the car world everyday and take too much for granted!
Drag Racers and Street Rodders that wish to run as much surface rubber on the ground as they can "Tub" their cars (widen the wheel wells to the inside). This allows them to run wide tire for traction.
But, trailers work way better the thinner the tire you use.
Your wheel tubs look like you could run 305's in them?? When what you really want to run for tires, are 155's to 185's (I run 165's on Miss Piggy)which are more suited for trailers.
Thus, if you "De-Tub" (make narrower" your wheel wells, from the inside, enough to accommodate the width of your new tires. The you will gain space inside your teardrop.
And fro your pictures this will be at least 6" (3" per side) or more! That is a lot in the TD world!
And by the looks of your wheel tubs they are beat to crap and should be replaced anyways.
They look galvinized.....so you won't be welding them!
But a cheap and easy fix is to narrow them to 3/4" larger than you want them (make sure you have the correct wheel clearance to allow for the sway you will get with that long spring set up) then cut out a piece of 3/4" plywood the same radius of the tub, and screw or nail it in place through the top of the tin. Seam seal everything, Rockguard the insides of your tubs and paint!!
Easy Peasy!!
Enjoy the extra room for whatever you wish to us it for?
Good roads
Brian & Sandi