Fund, I've been thinking further, and it looked (from the pictures) that your tank was in direct contact with the hull - is this correct?
Definitely make sure you have some space to allow for bilge slosh on the new tank. Just an opinion, and I'm by NO means an expert, just something that seems logical.
As long as you've come this far...
Maybe lay in a 1/2 spacer to set the tank a little higher, and allow water to escape from under it - and the underside of the tank to dry out.
If you want/need, you can send me dimensions of what you want for the the new tank, and I can Autcad them for you for your fab guy and send you a file.
Might work better than scribblings on a McNasty's napkin:willy_nilly: