I have the same carb on my 84 and as you said there is know "ported" vacuum as we have come to call it. The port where it apears to have no vacuum actually dose but only about 2.5" max. that wont work the vacuum advance.I have a vacuum amplifier that I am going to try and see if it will work but haven't got there yet.
If I were you I would leave the vacuum disconected and advance the timming untill it just starts to spark knock under load then back it off just untill the knock goes away. I did that with mine and my dist has a light spring and no heavy spring. Mechanical advance all in by 1800rpm I have it set for 34* total and that comes in at about 20* at idle. I get 15-16 mpg. I would guess about an extra 1-2mpg if I could get the vacuum working.
By the way when you get your 440 in don't even check your millage.