Could it be THIS simple?

[PKRWUD]

You've got the right idea.

I'd like to know your vacuum at idle, in Park. It would be really nice to know what your vacuum is when you feel you're losing your top end power, too. I'm guessing it's between 7" & 8".

Either way, an 8.5 PV will suit your set up better than a 6.5 PV will. May not solve all your problems, but it's a step in the right direction.

Throttle response has nothing to do with vacuum or mechanical secondaries, it's in how the accelerator pump is set up, the idle mixture, and the throttle plates. The reason your car should do better with a 3310 is vehicle weight and an auto tranny. That combo is difficult to tune mechanical secondaries for, whereas vacuum secondaries are tuned by engine demand. You just fine tune them with jets and the secondary spring.

Are you experiencing a bog, or it just feels like the carb runs out of juice way too soon? If it's the later, a 3310, properly set up, should solve it. In the mean time, an 8.5 PV will help.