Oh what a day...

pishta said

My friend had a Mccloud hydraulic clutch in a 65 Barracuda. Same thing, firewall flexing. So we looked and came up with a slant six transmission support bracket from the shock tower to the firewall. Worked and later found out that others did the same thing.

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Yes put a strut (threaded rod ) to clutch firewall area with large washers on each side to capture firewall
flex and bend other of the rod to a 90 deg angle and
attach to the inner fender. Adjust the nuts on the
firewall side for no firewall flex. That little flex movement makes a big difference in clutch performance. I don't know if there was less flex when the cars where new? or if its a wear issue.