No I used the original diesel tank location and had open access to everything. Plenty of people have done what you are looking to do, it's standard on a hard boat to have to cut out decks to replace tanks. Check out the boat building forum I have used, www.bateu2.com
. They build and repair boats. Lots of good advise.
You might call zodiac and see if you can find out a bit more about what's down there before you start cutting. Make sure you have enough room. With fills and pickups it knocks off a few inches of space.
Speedy tanks in NJ made mine. Great product and service. Pricing was good too although shipping hurts but I thinks it's still better than CA options if there are any