Best Carter 4 Barrel Carburetor for a stock 273?

If you are keeping the "2-barrel, stock cam", a 500 cfm carb will give good response and better mileage. If you think you may need to change carb jets, then get one with easily replaced jets. If it is an automatic, you probably won't use the secondaries very often.

Are you going to give it an exhaust system that will let it exhale better?

Make sure the timing is optimal; you can develope your own advance curve with a dial-back timing light and a plot of a few points with just mechanical and with vacuum advances. Compare that to curves for a factory, 4-barrel engine and you will be able to see if you have the distributor set up correctly. Tuning is critical to get a properly running engine.