Gas caps?

^^THIS^^ Completely stupid changes, it was related to the evaporative emissions changes through those years. Also important is what changes, if anything, that you did to the vent system. Any car made 70-ish and later with the evap emissions used a SEALED cap known as "pressure vacuum" which is NOT VENTED but rather just an emergencly release of pressure or vacuum in or out of the tank. THE VENT on those cars is a 1/4" tube running up into the engine bay which interconnects with the "carbon can" evap vent system. You MUST have that tube in place and if not tied in, then "open" up on the firewall to vent the tank. You can also plumb it into the air filter bonnet to reduce the chance of vapors.

I have no ideas on getting a proper locking cap in this day and age of "non" auto parts suppliers that we have.