I just installed the Summit Carb, 600 vacuum. And it's great. I haven't turned a screw on it, it worked out of the box, a claim I've seen often but never experienced. Fine idle, EXCELLENT throttle response and drivability, and feels powerful though I haven't measured. Feels like +50HP from seat of the pants...
It should be noted that my previous carb was 1850 Holley 600 vacuum that had been on a shelf for 10 years.
If you buy the Summit carb, remember to buy/install proper fittings for fuel inlets, the included chrome pipe is leaking even after tightening a lot, and I read on the web others have the same issue...