i say the carb is to big because even a high performance 350 will only pull 640 cfm of air the bigger ventures only slow it down i say this because i had a 750 on my 350 and my setup was more radical than that and i had the same problem so i went to a 670 and not only did it fix it doubled my accelaration and the only reason the Q-jet had 750 is it had really tiny primarys which kept the air moving fast when the secondarys open but you don't have that luxery with a holley and plus if it was made after 92 it has power valve blowout protection

 

GM ran 780 holleys factory on the 350s and also I believe the 302s. Ive ran through the cfm calculators and am really not convinced they are the best way to go, I dont believe the model takes into account all the build characteristics on each engine. Would a stock smog headed engine be capable of flowing the same amount of air as an engine with higher compression great flowing heads larger cam ... no its not ..but we would use the same formula for each engine. Carb flow is rated on 1.5 Hg of vacuum which is actually restrictive should the vacuum drop off(less restrictive) so will the capiblity of the carb to flow air that is why you see some races run larger cfm carbs Ibelieve the carb will still operate at below 1Hg . My other point refered to thedual plane manifold verses a single plane the formula doesnt take into account the manifold type even though the dual plane only allows each cylinder to see one half of the carb while the single plane allows each cylinder to see the entire carb. To calculate the carb size with all the physics going on in theengine might be able to be done but not by someone with my brain lol. Heck Barry Grant even says not to rely on formulas. Also this is a quote from car craft "Grant says a 350 Chevy with a dual-plane intake can usually use a 750 carb;" What exactly are the build details of this engine??

 

you can run a 750 just fine if you advance the timing a bit but also don't expect to much power below 3500 rpm i have built plenty of small blocks with carbs between 600- 750 and all of my 650s made better power then 750 at a lower rpm yes the 750 made better power above 3500 but not by much and it isn't worth the trade off of the low end and also if you have stock lifters expect them to give out around 5500-6000 so that is only about 2000 rpm that the 750 will be a factor uposed to the 670 from about 1500-6000. but that is just from my engine building experience

 

I would have to agree that in the lower rpm range the smaller carb would be a better choice I just dont believe a properly set up 750 on a dual plane manifold would cause the back firing problem.