We are using a 71 flip gas cap on The 66 and the twist cap under it. The problem is there is no room for a locking cap and we were concerned with possible gas theft.
I looked for those coil inserts from the 1970's that everyone put in the filler necks but nobody makes them any longer. We decided to use 1/4 inch stainless steel wire mesh and inside the filler neck and secure it with pop rivets.
Here is the link for pictures if you are interested:
Gas tank anti syphon screen
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Christophers 66 Mustang Restoration and Modification
A code 66 coupe
289 4v, Roller Rockers, Electric fan
Performer & 650 Edelbrock
MSD ignition
Tri Y into 2.25 Shelby side exhaust
T5 conversion
Hydraulic clutch
Granada Disks