Rebuilt 273 won't idle

Yep, it sounds like you don't have enough vacuum to hold the power pistons down and it's just to rich when it idles down, you probably need lighter springs in the power pistons. Does it have a vacuum advance on the dist, unplug it and see if it will idle. Shut the throttle plates all the way and turn the idle screw in about 1 turn after it hits the throttle shaft, turn the idle mixture screws out 2 turns, disconnect vacuum to dist and see if it runs.