The '61-'63 Kelsey-Hayes boosters used a rubber boot through the firewall with a rectangular opening for the bar that goes from the pedal arm to the bellcrank - same function as the one pictured for the Bendix-style booster. It may be a place to start.
Alan H. Tast AIA, LEED AP BD+C
Technical Director/Past President, Vintage Thunderbird Club Int'l.
Author, "Thunderbird 1955-1966" & "Thunderbird 50 Years"