Holley Carb Jetting Help, Lousy Fuel Mileage
we can't find a vacuum advance can with less than an 11 stamped on it. .
Been awhile since I tried to get a vacuum can from the usual Chinese suppliers, yo know, NAPA, O'Reallys
If you are running a Mopar breakerless dist. try asking for an advance for an EARLY SB, like a 67 273. My book shows they are around 8* distributor degrees, which would be 16 at the crank.
Even the 69 book shows the 69 340 is as much as 10* (dist) so 20 at the crank.
You either need to run ported vacuum. Some guys say run full vacuum at idle, but if you do, you'll have to completely rework the curve.
Frankly, I'd be looking for a small spreadbore, even if it means using a spread/ to square adaptor.
Hell, even my old 440 did better than that around town!!