Rebuilt 273 won't idle

Turn the curb idle screw out until it calms down for starters. Put a vacuum gauge on it so you can tell what it wants. Once you get it to idle at 1000 rpm, start tuning the idle circuit screws watching the vacuum gauge. Cant speak to a stick car but an automatic car with your build should idle about 800 rpm in neutral and 650 +- in gear. Could be it wants a little more initial timing as well but you should be able to get it from where you are.
Good Luck!