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Steve,
I agree with MidOHasp, even with 100% VE at 6K RPM, 625 is maximum draw. So a 650 would give you plenty of headroom.
If turning back the secondaries helped the bog, you may be over carbed at 750 CFM. When the secondaries open too soon/fast there is not enough air flow over the venturis so it leans out - the accelerator pump offsets that but won't result in as even a mix as the venturis. It may also be, as suggested, the accelerator pumps, particularly if the change occured suddenly. Remember too that cold air (this time of year) will make for a leaner mixture, aggravating things if it is marginal to start with.
Another possibility is to use vacuum secondaries with the quick change diaphram spring kit from Holley. You'll get great drivability for street using a stiffer spring and can change it to a lighter spring for track (acting much like a mechanical secondary) by just removing a cover held on by two screws.
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