Dan Thompson wrote:
Personally, I'd say the placard is all you need, but only if you are sure
the STC for a plane like yours is just a paper whipping excercise. If so,
I'm sure you don't even need the placard to fly on autofuel.

For a 172 it's only paper whipping. However to be legal you need the
337 filled out and sent in, the logbook entry, the clamp, and the
placards at the fuel fillers.