less so vertically because of the filler neck in the rear tub.
FWIW - you could consider doing away with the rear filler; If of course a bigger tank is of interest. You could leave the current external cap/part and just have a top mounted filler, far easier/quicker when refueling.
You might be surprised by the quote ( in a good way ) and if not, probs worth ringing around locally.
I talked with the guy who runs the local forge last week, and he's scratching - umpteen projects 'on hold'.
If you make it easy for whoever does the job; clear idea about measurements, what you want and maybe even sourcing the inlet/outlet so all they have to do is clean/purge the tank, cut and weld - you may get a 'result'.
Upside of taking the back off to take the tank out is that while its out you could clean the crud and give it a refresh.