Hi Brad. Welcome!
There a a number of fuel tank replacement threads here on the site. This one in particular (on a KK 42) outlines the entire process including a tally of costs:
It's typical to replace with multiple smaller tanks as they are easier to install.
While each boat is different GBs have a reputation for tank replacements - so much so that some boatyards specialize in that work.
Seems like removing a 25 gal tank would be pretty simple.
New tanks can be fabricated or you may get lucky with off the shelf tanks. Again, combining multiple tanks per side reduces the need for an engine pull.
as far as the largest tank without engine removal, completely dependent on the ER layout.
Hopefully someone on the forum can give you some leads on shops specialize in GB tanks who could probably give you a good idea of what's possible and involved over the phone.
Finally, I'd expect for a BIG discount for a GB that only had 50 gals on fuel capacity.