I've run a Holley/Weber on my 57-235 in an AD truck and would highly recommend the setup. As for any great increase in mileage I don't think you're going to be overwhelmed by that partiular part of the change. I think a properly tuned Rochester would provide as good fuel economy. This however, is where the H/W modification shines. My experience with 50 year Rochester carbs has been that many are excessively worn in the throttle rod area, the air horns are warped, they don't seem to provide a good idle, and have flat spots and hesitation that effect higher speed operation. All of these conditions are eliminated with the H/W.....right out of the box.
The installation is straight-forward, the linkage kit is easy to assemble and adapt to the existing linkage, and tuning is simple. I would recommend the 12v electric choke upgrade ($12.)
Fuel mileage is a very subjective area....the state of tune, driving habits, gearing, and terrain all have a major influence. From my own experience my overall mileage with the single H/W on the 57 engine and the dual Carter /Webers on my 1960 modified engine average between 18-20 mpg.
To me, the big plus with the H/W setup is the improvement in overall performance and the loss of the gas smell from the engine compartment as well as in the cab. I've documented both the installs in my Webshots albums if you'd care to browse.
Dave
