Stu...
those of us with the Hunter 34's have fuel tanks that are of poly construction (probably roto-molded), triangular in shape and set in place before the hull and deck are mated. It's located in our starboard side cockpit locker that has an odd shaped opening into its depths (about 14"x16") that is too small to either pass the old tank out or put a new tank back through. I can't squeeze into it after a light lunch!
Some owners have gained access through the bulkhead that separates that compartment from the aft cabin to accomplish a replacement when absolutely necessary. It does require a good bit of carpentry work by someone semi-skilled to make it not look like a hatchet job and you're working in about 24" of verticle space.
There are no aftermarket equivalents for the original tank. However, unlike metal ones, there are no baffles inside to create cleaning problems so cutting it open and cleaning it out thoroughly can be done (best accomplished by someone dynamically heigth-challenged!). Cutting it open, cleaning it out, and resealing it, even with the expensive SS port like I used, is an economically viable solution.
From many of the different boats I've been on, the easily replaceable fuel tank is the exception rather than the rule. The history of leaky metal tanks is such that I'm glad mine's a poly one. One of my dock-mates has a center-cockpit Hunter with a metal tank located under the aft cabin bed deck--one of the easiest accessible tanks I've ever seen. Every boat is different.