How about some pix. Rusty tanks are a real problem. If your fill is on the front deck and you don't want to do a bunch of filling, sanding, priming and paint you may want to have a new aluminum bow tank made. I know it's a bitch and pricy but all the rest of the issues mentioned might make it the least expensive. That being said I like saddle tanks.
Bad tank story... My dad went into a partnership on a 32' Cris Craft fishing boat years ago they named it the Hard Way. After a few years of trying to get it all cleaned up and seaworthy the rest of the partners wanted out so they decided to sell. one week after it was sold the guy blew it up burned to the water line and sunk just down the coast from Newport breakwater. They told him before hand that it needed work on the tanks and to NEVER TRY TO START IT WITHOUT RUNNING THE BILDGE BLOWER FOR 5 MIN. He didn't and it tossed him 30 feet up and 30' out from the boat when the ingnition set off the fumes. He was wearing a wet suit and was suprisingly uninjured.
