What cam should I get?

It's a 273 with non-adjustable rockers?
Pay close attention to what everyone has been telling you about the .484 Purple Shaft cam, because it will make your car a total pig off the line. That's not just a guess, that's the reality of installing a cam that's WAY too big for that size engine.
That cam will bleed off way too much of your compression, effectively killing the bottom end. Just because the cam physically fits in the block doesn't mean it's right for your particular combination, and in this instance, it's all wrong. If you know which shop built the engine, why not go see, or call them, and try to determine exactly what the specs are on your current cam. Once you have the specs, then we can make some informed recommendations for any changes.
I, for one, believe that there have been more nice street machines ruined because of over-camming than for any other single reason.