1- JD Z, the "tech" might be basing his opinion on the "Holley" chart along with his view points, experience and what he see's based on your engine and set-up. As a guide, Holley does state a 600 cfm for 350ci up to 6500 rpm, 650 cfm up to 7,000 rpm and a 700 cfm for 7,500 rpm. You may get them to check the carb to see if it's been re jetted. The 4779-9 should have a 71 jet primary with a 6.5 Power valve, 81 jet in the back, no Power Valve. 2- You will not be losing anything with the 650, all else being equal. You may in fact gain throttle response at most rpm's, and drive-ability if the carb is tuned right by a pro. PS : There are great carb companies out there that could help you figure out what carb is best based on engine size,RPM, and components being used and expected results (Pro Systems/AED/Quick Fuel to name a few). Based on my experience, (SBC, single carb) the quick rule of thumb I use, and have observed on many occasions, is you need approx. 1.3 to 1.4 cfm per HP. That would mean a 650 CFM would deliver well on a 465 to 500HP engine. FWIW, I have done the test on my engine, going from a 750 Holley to a 650 Holley, all else equal, I only lost only 6 tq and 8 hp on the top of the rpm band. My engine used 700 CFM. Hope this helps out.