gas cap

GTS Dave said
The only thing is the chrome cap doesn't have the 'pressure relief valve', it's just a plain cap.

That's not going to do. You need a pressure-vacuum cap.

It does have the rubber seal which should suffice a vapor barrier.

The rubber seal is to prevent liquid gasoline sloshing out (and water drizzling in). It does not substitute for the controlled in-and-out venting provided by a pressure-vacuum cap.

Read up on it here.