My 273 turned out to be a 318...what did I buy?

you should check to see if an inertia switch was wired into the fuel pump circuit

This!

Initially when swapping to a Magnum I ran an electric fuel pump. After having one quit on me I bought the cam snout extension and went back to a mechanical pump. Another reason was to get rid of the pump noise...my Autistic grandson wouldn't ride in it because of the noise. I never got around to putting in a inertia switch and that was always on my mind when driving. No worries now!