You might find it in an owner's manual - some of them had parts lists. You may be about to find it/download it from GW's site. Otherwise contact Grady directly. Personally, I'd VERY wary of relying only on a part number without measuring. That's a big gamble with a lot of money that's "all on you".
I'd do anything I could to access the boat - either put a door in the shrink wrap or even remove/redo. Even if the boat was 10 hours away, I'd make that trip.
You "might" try and see if the newer plastic tank used in the newer Tourney's would fit - that's a better long term solution. Or, heck, measure the coffin and see what Moeller has available "off the shelf".