All Marine Traders are unique so what applied to mine may not apply to yours.
Big question up front is do you want to keep the original configuration of the tanks or will you accept some creativity and compromise to have a better but unique tank setup. Second question is are you going to have a boatyard do the work or are you capable of a project like this? Third question is money. It may answer question number 2 for you.
If you want the original setup you need a boatyard that can handle removing the engine and transmission out of the cabin sole hatch and out the side door. Then they can remove the tanks intact out the same door. Send them out to a fabricator and have them duplicated. Do both, you don't want to do this again. Replace the tanks, replace the engine and transmission, cleanup the engine room, refuel and go for a sea trial after dropping off the check. I went this route in 1994 and it cost $8,000 and took all summer. Please adjust for inflation and your socio-economic area. Figure $10,000 to $15,000 in today's money. But you do save on hand-cleaner and chiropractic fees.
Leaving the engine in place and removing the tanks by cutting them up into small pieces with a Sawsall is another an option. Buy 3 or 4 smaller plastic tanks and build a robust 'cage' to hold them in place and connect them together. Depending on configuration, you will lose some capacity but gain flexibility with this arrangement. You will also save thousands, perhaps tens of thousands of dollars. You can work at your own pace and be able to say ' I did it myself'. If you do it right you won't even affect the resale value.
Unless you live in New Zealand or Australia, this is the time of year to do a project like this. You can spend all winter under shrink-wrap doing it yourself or negotiate lower off-season rates with your boatyard.
34' Marine Trader