The 1964 427 Hi Riser automatic cars used a modified Lincoln HX transmission and then later the Galaxie Police MX was an optional replacement around June 1964. The converter used with the HX was a 12" Galaxie as used in the production MX transmission. There was a secondary GMA Weld on the Cover studs, but Ford already used a secondary arc weld in production, so it really wan't anything "special". Ford also started investigating higher stall converters the same time they released the MX as a replacement behind the 427 HR engine.
The C6 was not yet in production for 1964 model year. Eventually Ford sent NHRA a letter to permit a production Cobra Jet C6 as a replacement for the obsolete PCA-F HX transmission.
As you indicated the engine specifications would dictate if a higher stall converter would be more beneficial.
This message has been edited by tbolt2 on Jul 10, 2012 9:34 AM