The 1400-series Performer carb should be good for even a mildly-build 350 engine without the need of a spacer. I have a crate 350 motor in my '60 Impala, bored .030 over, mild performance cam, and Edelbrock Performer series dual-plane intake manifold. When I purchased the Impala, it had a 1400 with manual choke (1405) which I converted to electric choke (1406) to improve driveability for my wife and daughters. (They still can't master the manual choke on my stovebolt!) My carb is calibrated to center-mass on the chart. I am quite happy with that setup...good economy and driveability, plus the ability to start a tire fire whenever I want!
I used an online calculator (there are several available), and with my displacement, at 6000 rpm (I don't want to rev higher!), and a volumetric efficiency of 85%, the carburetor CFM rating required is 524 CFM. For grins, I played around with the calculator a bit: If I rev to 6500 RPM and my intake/head combination attain 90% volumetric efficiency (!), it calculates to 600.9 CFM.
Carbking is way smarter than I, and may have other opinions...but... I don't think you're going to gain anything noticeable with a spacer. The largest gain will be to ensure your carb is in good condition, and calibrated properly (based on your application); rebuild kits and calibration kits are readily available, and not too expensive. Next would be ensuring the intake is optimized for the carb, engine, and application. I don't know anything about the Weiand intake, so I'm no help there.
-Kevin
