A lot depends on which cylinder heads you have. 68 and newer open chamber Pontiac heads tend not to need as much total advance as do the earlier closed chamber heads (especially the 67 GTO '670' heads). The only way to find out what your engine likes best is through dyno testing or testing at the track with an accurate set of E.T. clocks. Open chamber Pontiac heads generally tend to like something in the vicinity of 35-36 degrees total. (That's at wide open throttle, under load --- which renders the vacuum advance inoperative). Whoever told you that the vacuum advance mechanism will add "too much" at part throttle, light load operation is probably mistaken. Set the car up for what it likes best under load (see above) then reconnect the vacuum system. If it doesn't run hot, get hard to start, or otherwise act up --- stop worrying about the vacuum. As far as carburetion. Nothing beats a PROPERLY set up QJet on an engine like yours. 650 cfm would normally be borderline not enough on a high output 400 in say a GTO but might be sufficient for a 'normal' 400. Also, this carb is not truly mechanical on the secondary side. Yes, the throttle plates are mechanical (so are the ones in a QJet) but that's not the whole story. There is an air valve in the upper half of the carb that is under spring tension and will not open until the engine is generating enough vacuum to pull it open. Tuning how quickly this valve is allowed to open is how you set one of these up. You have to hit the sweet spot on opening rate such that it's not fast enough to dump too much air into the engine too quickly (causing a big old bog) but fast enough to let the engine get all the air it wants.

If it is a 650 Edelbrock Carburetor that you have, then it should be the AVS series (#1806-satin or #18064-eudura shine) which has the capabilities to adjust the secondary opening rate. The process to adjust it is quite simple. Vacuum advance is designed for economy and really not needed. Usually 36-38 total timing works fine on many engines. Hope this helps