For performance and some fuel economy I have had great success with the Holley/Weber from Tom Langdon's Stovebolt Co. It's a 2 stage 2 bl and Tom sells rebuilt ones for $65 or at least he used to. I have a pair of them on my 261 and it runs very well and still gets 20+ mpg. Forman is right that a cam and exhaust will help but you will be surprised how well those engines respond to simply adding carburetion. I ran a single one of those carbs on a 235 in my '62 Biscayne and it worked very well.
