so the reason for leaving a good 12" before the transom is to leave room to work on the bulkhead? or in case of collision?
and your guess is because of the way the fiberglass tanks were mounted lead to a stress crack over time that is now leaking?
To leave room to work, and also to mount your new tanks on only one stress axis. I didn't even consider the collision aspect, but it's not a bad concept. It is my opinion that the way your tanks were mounted probably led to their demise, although fiberglass tanks just suck anyways.
I don't know for sure, but I would guess that a pair of new aluminum tanks will cost you around $600. Getting the old ones out, making the new bulkheads and installing them should take a competent glass guy about 4 hrs (avg shop rate $75/hr) if he didn't have to pull out the interior or do anything strange.
Add a little more to that if you wanted the shop to actually install your new tanks so they were ready to use (fill/vent hoses,fuel lines, hardware, etc...)
BOAT= Break out another thousand......