Well I can't help but throw my two cents in. First a few questions: You put that carburetor on 15 years ago, did it bog off idle ever since? Was the car running well before you replaced the carburetor? It's a 67, does it still have all the pollution control stuff attached? It's 49 years old, chances are it may have been rebuilt in that time. Do you really know what camshaft, pistons, bore, head/block machining etc. has been done to or in the engine today? What is your rear gearing? What tranny do you have? What diameter tires?
First you have to know what you have and what your intended usage will be before you can really choose a carb. All those carburetor selection charts will tell you the max cfm you need for a particular rpm not necessarily what you will use. If you are over carbed with a Holley 670 you will be over carbed with a 750 or 800 CFM Rochester.
That Holley dual inlet vacuum secondary is a nice carburetor, you should be able to tune it to your car. The Edelbrock and Rochester carburetors are nice as well. If after all your testing you feel you need a smaller carb there's a Holley 570 Street Avenger as well as a 500 or 600 Edelbrock Performance Series.
There are many reasons for bog on acceleration, not all carb related. May I suggest you check all possibilities before you take that 670 off and replace it with a new one? We wouldn't want you to spend time and money installing a new carburetor just to have the same symptoms. First check you induction for any vacuum leaks, leaks will lead you to make carburetor adjustment to offset the the effects of the leak and in turn give you poor off idle throttle response and incorrect vacuum advance. Do a compression test, the results will inform you of possible internal engine faults such as worn rings, burned valves, worn camshaft etc. Check you timing and operation of the mechanical and vacuum advance. Check plugs and wires, points, distributor cap and voltage to the distributor when running. Connect a vacuum gauge then check needle movement while running, that may lead you to an otherwise undetected fault. Replace your fuel filters and test your fuel pump. Check your exhaust for blockage. And of course check all those things I've forgotten to mention.
After checking and correcting any other problems start diagnosing the operation of your carb. Measure the float height, check the operation of the choke, the accelerator pump, throttle linkage operation, throttle plates achieving wide open, operation of the secondaries. The secondaries opening too early would be a main concern. The secondary circuit can be tuned with the use of different springs but operation of the secondary vacuum system should be checked. If you cannot inspect all the carburetor functions yourself do you know anyone who would give you a hand? If not the Holley help line may be of service.
Now to the camshaft and heads, as stated above the car was built with the camshaft, heads, manifolds, exhaust and gearing to compliment each other. The stock camshaft and heads were not made for high rpm high HP. But you want a reliable street cruiser. The camshaft won't make big power at 7000 rpm but it will supply you with good torque at lower rpm, just what you need in a street cruiser. As for the heads, it's true big valve heads with large screw in studs, nice fast burn combustion chambers and high flowing ports are sought after for max power engines. But most of us don't run our cars on the racetrack and don't have all the other engine components to support heads like that. For your engine a head that has more velocity is most likely better for you than a head that has high flow volume. Your valves and springs are not going to cause your studs to dislodge and they flow enough to feed your street cruiser 350.
