Well...as I recall some 250 engines used ported vacuum and some used manifold vacuum for the distributor advance. And I don't know which year/model of 250 was which, but I do seem to remember ported vacuum was used on the ones which had smog pumps. Do you know the year of your engine? Has the distributor been changed? That would help here. In your carburetor, I believe you will find one of those vacuum ports will be at or slightly above where the butterfly rests at idle. That will be ported vacuum. And you will find one where the vacuum connection is below the butterfly (and this will be manifold vacuum).

The difference is important and it all has to do with how your distributor was designed. If you connect manifold vacuum to a distributor designed for ported vacuum, you get full advance at idle and then lowering advance (lowering vacuum) as you step on the gas. Your distributor probably wanted to see the opposite. If you have a distributor that is looking for manifold vacuum, you have higher vacuum at idle and then as you drive, your rpms increase and the centrifugal advance (weights) in your distributor sling outward and more or less take up the slack and increase the advance for you. So you have to match the two. As for the third connection, I don't know. Didn't Tom send you any info with the carburetors?

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end