When you get a chance & if you can get to it, measure the height of the forward side of the tank where the fuel PU is located(deepest portion). I had found some old drawings supplied by another owner that showed the tank at 11.75" tall but when I measured mine it was 13.75" and obviously was not coming out the lazarette now. It could just be a tank capacity purchase option and if yours is the shorter we'll know for sure. Maybe that's why my plywood board seems to be narrower ? Don't think my gauge is accurate either but when I pumped out all the fuel last week I got close to 23 gallons. There was about 1 1/2" of air on top when I started so close to 25 g for sure. All that said I am going to try and repair tank in place. I have another post regarding fuel tank issue and probably post process there when I start in spring.
I continue to be interested in this thread, and it's complexity. I think we agreed earlier that my hull #637 was built in Oct. 1984, so after the change you reference. But I am almost sure that I have the 25 gal fuel tank rather than the 19 gal that you think comes in the later models. So I think we all need to be cautious to not assume that either earlier or later is consistently different in all aspects--it might have depended a bit on what supplies were still available in the Ericson inventory as they were making the change, though likely any change affecting the hull/deck mould would be consistent, I would think.