Well best one is a toss up. I guess it would be one that you can keep in tune. anything that causes a miss fire means you lost 1/6 of your fuel economy.
I was getting 20 mpg with my 1 bbl.
In theory, the Weber with the small primary should give you the best but there are a lot of Webers out there getting worse mileage than I am. But who want's to go back to a 1 bbl.
IF it's really important then look at FI either a throttle body or MPFI that you can tune electronically. Of course you will need a good, tight engine to start with.
There is another Canadian here on this forum that's getting 24 mpg with his BBD.
Post a picture of your hoses and what they are attached to, maybe we can figure something out.
anything like this? :
This is an 82 vacuum diagram.:
I have an 80 with all the pollution stuff attached. May that will help.