I cleaned out a similar tank some years ago. It was an awkward fit and the access was worse but we were at the point where one more fuel
mystery would have left a contest to see who could rip out the damn thing first.
So...remove battery cables
and steering cables
. Literally remove fuel tank, coax it out of the lazarette. (Damned good thing there wasn't much fuel in it.)
Then, remove the fuel sender, which gave us some access into the tank. Try to scrub the walls as best possible, because there was black biomatter bound to the walls, and that's the stuff that comes off from time to time, especially in rough weather
, and comes back to plague you. When it is as clean as you can, see if you can take it to a radiator shop or someplace with a steam hose to try cleaning it out even more. Then, pour a box of coarse kosher salt
into the tank. Honest.
This is an old trick. The coarse salt
is abrasive. Now some brave soul needs to lift
and tumble the tank until you're exhausted, so you are tumbling the tank with an abrasive to remove anything you missed. When that's done, you fill the tank with water (preferably warm) which conveniently dissolves any abrasive that's left. Duh, that's why SALT is good for this! A good rinsing means no abrasive is left in the tank, and leaving it out in the sun to dry out is all you need.
You'll have a clean tank to reinstall. The trick is, to try keeping it that way. PS tested using a moisture absorber on the tank vent line, and found that can really help keep water out of fuel tanks