The gasket that you are looking for that goes behind the tail light lenses on the '67 LeMans are a unique gasket that is not being reproduced. Most used ones crumble when the tail lights are disassembled. I have found a couple of good ones by buying used tail light assemblies where the gasket was still usable.
Several years ago I found a pair of NOS ones that went on my '67 LeMans.
Also, I don't believe that strip caulk would be a good substitute in this application.
I would try to find some good used ones.
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Bob Kazian
Greer, SC
[email protected]