The issue is that the current carb is mechanical secondary per the list number you posted. If you go with a 750 vacuum secondary, the secondaries will only open as needed vs what you have now that open up as much as you tell them to. I've run an 850 vac secondary on a POS Buick 350, and when tuned properly it actually gained me almost a tenth at the track over a 750. Both were tuned by wideband. I'd just put a 750 vacuum secondary carb on it, and tune as required.