[Excaliber]

So why did the Competition Cobras run a single four (dual fours DID make more horse power)?

I would speculate it was because the single four had a 'center pivot' float that kept fuel from sloshing into the mouth of the carb under high cornering or braking g forces. The center pivot fuel bowls are too long to mount two carbs back to back on a 2X4 intake.

All though Shelby did put two fours on a GT40 scheduled for Le Mans in an effort to find more horse power. Not sure if it stayed that way for the race?

Medium Riser heads on the SC? High Riser heads were discontinued in 1964, but would have been available if an an owner wanted them.