Recently had to replace a tank. We carefully pumped the old tank as dry as we could get it. Then we cut the old tank in several smaller sections and then removed the old tank, section at a time. Carefully measured and then re measured the replacement tank. Be sure you have looked at every possible angle in order for this tank to be installed. We were able to replace the old with a single smaller tank, that we could snake into the opening and fit in the proper place. We sacrificed 8 gallons od fuel , which for Sue Marie is a non issue. Before ordering the tank, we built a model to the measurements , just to be sure.
I agree with Capt. Bill , stay away from the Bladder. It took a little bit longer to do the model but well worth the reduction in stress. Our Prairie 29' was originally built with two 50 gallon fuel tanks, made out of Aluminum . We have now replaced both tanks with Aluminum, they each hold 42 gallons. We are happy and we did not have to remove the engine.