Here’s an ebay link to what looks like the flat cover.
http://cgi.ebay.com/ebaymotors/1955-1956-1957-Ford-Thunderbird-valley-pan-292-312-E_W0QQcmdZViewItemQQhashZitem4a9e6cf3dcQQ
itemZ320485520348QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
I’ve seen plenty of modified stock covers where they were simply notched lower in the center of the cover so that the Ford dual quad intake would clear so that’s an option also. As AussieBill mentions, the factory recessed covers are in short supply. There was someone reproducing these for awhile so some of these may still be available. Early on I use to see the factory recessed covers on two barrel cars and didn’t realize the significance of them until much later as they were dual quad engines having their original 2X4 intakes removed.