If you want to go the cheap route....
I have a 77 REPU and could not find rear glass gasket so i did this.
Find a gasket in decent shape at a junk yard and recondition it with baby or mineral oil. It works great.
Got this from Gborch. Thanks for the idea Gary...
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'77 REPU
Stock 6-port
T-2 tranny
Weber IDA 48
Alum Flywheel
3rd gen Torsen in rear end.
DLDFIS ignition