273 distributors vary greatly for mechanical advance. Initial advance can be 10* BTDC to 5* ATDC ! There are specs for them if you have the part number of the distributor. If not, you can get a ball park number by doing a little bit of checking.
Place a distributor in a vice horizontally. Hold it by the shaft. Mark the rotor tip in the center. -- mark the other end of the rotor in the center. Cut a piece of cardboard [cereal box ?] to the height of the rotor centerline. Rotate the dist. shaft so the rotor is parallel to the workbench. Tighten vice. The rotor is spring loaded in one direction. While using a taller piece of cardboard that sits on the bench, turn the rotor against the spring and trim the cardboard till it matches the angle of the rotor. This may take more than one try. Using a protractor measure the angle. Double the angle. This is your mechanical advance.
My unknown small block distributor ended up being 17* at the dist. So, 34* at the crank.
Initial timing at TDC is a good starting point.
