I have not placed a four barrel on a 270 however:
My shop truck for more than 400,000 miles was a non-GM 300 CID six. Offenhauser produced a 4 barrel intake for this engine.
Removal of the Holley 1 barrel (stock) and replacement with a Carter 9400 (400 CFM) resulted in better low end torque, much better power under WOT conditions, and surprisingly to me, about 50 percent better fuel economy on the highway. After the change, got 22 MPG at 70 MPH in a van.
(Opinion) Zenith made a much better carburetor than Holley, thus results will probably not be so dramatic.
Now, playing with some numbers:
The Zenith for the 270 is rated 215 CFM (2 barrel scale). This converts to 152 CFM on the 4 barrel scale.
The Carter 9400 is rated 400 CFM (4 barrel scale). This can be thought of as a fixed 200 CFM (primary side) and an as-needed 200 additional CFM (secondary side). Remember that Carter has an auxiliary secondary weighted airvalve. Even though you open the throttle, the secondary won't provide additional air/fuel until the engine actually needs it.
Thus the fixed portion of the 4 barrel is approximately 50 CFM larger than the Zenith. However, the primary side on the 4 barrel has two smaller barrels than the one larger barrel on the Zenith. This increases air velocity around the throttle plates at lower RPM making the small 4 barrel (with slightly more CFM) at least as efficient as the large 1 barrel.
Will it pay off financially? I cannot answer that, as I have no idea how many miles you will drive; nor how much you will spend for an intake. The 400 CFM Carters are out of production, and you will pay a collector price for one. With my 300 CID and 400,000 miles, the conversion paid off many times over.
And this post should not be contrued as a recommendation; simply the other side of the coin from actual experience on a similar engine.
Jon.
