Glen E.
My opinion is that the Offy and a nice 2 barrel would probably give the best results for your application. The one barrel just doesn't allow enough air into the engine to produce any decent power at sustained higher RPMs (which for a stock 250 like you have is 3000-3500 RPM).
The 2 barrel with the Offy would give you better low end torque, compared to stock, because of the additional airflow for the stock cam/head combo. And the 2 barrel (350 cfm max for your app) would not flood the engine with too much air/fuel.
There is a really good section of a book written back in the 1950 by "California" Bill Fisher (called "How to Hop up Chevrolet Engines") that discusses in detail (and shows a graph of) the power differences in a 235 with a stock 1 bbl carb versus a 235 stock with 2 1-bbl carbs. There was about a 12 HP gain in going with an extra 1 bbl carb. That's HUGE for a bolt on mod on an otherwise unchanged engine. Not only was there an improvement in power, they also note how the peak RPM curve moved up slightly. They added a third 1 barrel carb and there was yet another increase in peak HP as well as a slight increase in the peak RPM curve. The improvement in going from 2 1-bbl carbs to 3 1-bbl carbs was not nearly as great as going from a single 1-bbl to 2. The author noted that the increase in power in the 3 1-bbls was only significantly noticed once other power equipment (hotter cam, headers) were added.
That's why I think you'll get the most bang for your buck with the 2 barrel. On the smaller six, unless you're going to beef up the camming, the airflow (by way of headwork), the compression and ignition, then the 4 barrel would not compliment your engine as much as the 2 barrel.
-ftyler
