Gas cap dilemma

71 ECS (70 CA) caps are very hard to find and very expensive. I am selling one in the for sale section (with a filler neck and vent canister)
[FOR SALE] - '71 ECS ('70 CA) Gas cap, filler neck, vapor canister

Your best bet is to get a 72+ filler neck and replace the old one if you're up to the task. That can spiral though (need to paint the new cap, new filler neck to body gasket, possibly replace the tank grommet, etc). 72+ caps can be had anywhere and new ones with primer ready for paint aren't too expensive and can be found on ebay

I just did this swap recently. I went with a 75-76 filler neck which takes the same cap as 72-74 but it's the unleaded style. It provides the best anti-splashback benefits.