Try Dewitts if you are looking for a replacement. They will tell you exactly what will work best. They are a bit pricey, but well worth it.
The radiator really should never be filled all the way to the top. The system needs a little room for expansion. About 1-2 inches below the neck sounds just about right.
You may indeed have a radiator that is not up to snuff anymore.
The seals around the shroud, and over the top, are pretty important for good cooling, especially on a Rat engined 'Vette.
Do you know if your fan-clutch is operating correctly?
An overflow/recovery tank is a good idea on any car, imo...
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Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"