What cam should I get?

The mopar purple shaft cams are old tech designs, not bad cams but there are many better designs out there now. The smaller cams from the Comp Extreme Energy, Lunati Voodoo or Hughes will be a much better choice.

Also, remember a 273 is a small engine and it is not going to have a lot of power. The base 318 2bbl had more power than the 273 Commando motor.

BTW, you would defineately need to cahnge the stock valve springs to use the .484 purple shaft cam. Unless the cam manufacturer states that the cam will work with stock springs you really need to swap the springs to what the cam manufacturer recomends.