Hi Michael,
I don't think it's an original tank for a couple of reasons: 1. As previously noted, they never used foam at the factory. In fact SeaCraft brochures used to brag about not using foam, and they are one of the few boat builders that provided a removable hatch to allow for easy (at least on Sceptre's and Seafari's!) inspection and removal of the tank! They said it cost them more because then they had ventilate the bilge, but they did it anyway because it was the right thing to do. 2. Every SeaCraft I've seen had a thin (~1/4") piece of plywood covered by one layer of glass on the top, under the tank on top of the keelson. It looks like whoever installed that tank didn't know what they were doing removed that panel and thought the foam would suffice!
Some strips to hold the tank off the supporting panel are a good idea, but I think I'd use some sort of inert plastic like starboard or pvc board instead of wood that might rot. Don't use rubber - some rubber contains carbon that will create a galvanic couple with the tank and corrode it! And yes, the "right type of foam" is NONE!
As for changing tank size, I'd just give the compartment dimensions to the tank builder and let him figure out how tall it can be and still have clearance for the fill and vent hoses. The vent should be located on part of tank that's the highest when you're standing the boat refueling it. Not sure where that is if your fill port is amidships - the gas fill on mine is on stbd gunnel about 2' fwd of transom; vent fitting on my original tank is at front of tank on port side.
Denny
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