Tank to bowl seal for Eljer?

I have an Eljer toilet manufactured in 1974. It started leaking between the tank and the bowl (when flushing). After removing the tank (had to cut one of the bolts due to corrosion). I noticed the seal was really deformed and did not look like it was ever seated correctly. Looking at the tank, it is completely flat at the bottom. The bowl is recessed where the gasket belongs and where the tank bolts pass through. Trying the generic tank to bowl gasket, The tank does not want to tighten enogh to come anywhere close to the bowl. With nothing to hold the new seal indexed, it "pooches" into the opening in the bowl, and the tank tilts back. The newer style Eljer gaskets with the "ears" does not appear to fit well, and the rear feet on the ears do not sit on the bowl (extend behind it).

Is there a special gasket for these older Eljer toilets, or I am missing something obvious? With nothing to keep the gasket in place, what should prevent it from popping into the bowl opening, i.e. pinching out of the intended space? How thick should the gasket be? I do not want to crack the tank. I have three girls and fear clogging issues with a more modern toilet. I also have some issues with tree roots encroaching into the sewer line, so clogs are even more likely with less water flow.

Alll inputs welcome (constructive or not . . .)

eljer

Yes, there is a special Eljer gasket. It is essentially flat, but the center portion is thickened so it sits into the recess on the bowl. Any good plumbing parts store will have one. If you cannot find one come back and I will give you a "work around", but first try to find the proper seal.

Thanks HJ

I mistakenly went to two different big box stores thinking this was a universal application. If I would have called a plumber in the first place, I would have saved a couple hours of hassle, and a few days of down time. DIY often ends up costing more than doing it right!