Just use a conversion kit to convert the 4160 to a 4150 so you can jet the secondary side.. Go back to mid-range secondary vacuum spring. The 600 should give good signal and response on that 350. I'd bet the tuning is way off and the secondaries are opening too early, making the engine feel soft. A slightly too-small carb will be more responsive than a too large carb on the street. Install a wideband O2 sensor and report back.
My first Pontiac was a 350 in a '67 Lemans, they can run.
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I could explain all this to the girl at the parts store, but she'd probably call the asylum.
White '67 LeMans 407/TH350/Ford 3.89... RIP
Red '67 LeMans. 407/TH400/Ford 3.25
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