To address your questions simply: A 4V carb will deliver better performance, driveability, and potentially better economy - if you can keep your foot out of it! For street use, vacuum secondaries are the best choice. They will deliver more air and fuel, but only when your engine asks for it - not just when you jiggle the skinny pedal. Mechanical secondaries work best with manual transmissions and do poorly in stop-and-go traffic, delivering rotten mileage, stumbles, hesitation, and general fuss for part-throttle operation.
Holleys can make good top end, but they don’t offer annular boosters except on their high-end carbs. Annulars are absolutely superior for street use, delivering better fuel atomization, and increased signal sensitivity. That means increased throttle response, more average power (not just top-end), and better economy, since you won’t be spitting as much unburnt fuel out the tailpipe.
The old Autolite 4100s are one of the very best 4v carbs, but their flow tops out around 500 cfm or so, and even with the modest RPMs you’re describing, a healthy 351 could potentially run out of air. They are also hard to tune, since parts for them are both scarce and because they were never intended to be modified.
Instead, for your engine, I think I would go with the Summit M-series 600CFM carb. It’s a direct descendant of the old 4100, and has a lot of similarities, but it is much easier to tune, and uses standard Holley jets and parts. It has vacuum secondaries, and annual boosters too. With a price tag of just over $300, it’s mighty hard to beat. (for those of you ‘in the know’, yes it’s based on the ill-fated Holley 4110. But the 4110 was based on the Autolite 4100. So yes, they are closely related.) If you were going to do any mods with your engine, and possibly want to hop it up later, as Royce said, 750 is usually not too much for a 351, but the smaller primaries of the 600 will offer slightly better throttle response and economy.
If you wanted a cheap dragstrip terror with mediocre manners, a Craigslist or Ebay Holley would be fine. They are common as grains of sand on a beach, and at wide-open-throttle, properly tuned, they can probably even beat the Summit carb by about 5-10 horsepower. The larger fuel droplet size produced by the Holleys takes less room in the intake, allowing more oxygen and fuel into the chamber. Annular boosters typically deliver almost pure vapor instead of droplets at WOT on the top end, and that displaces some of the air. But for a car you intend to drive on the street, the M-series would be way better, delivering a few more MPG and better overall power.
And Yearby, dished pistons decrease compression (which theoretically lowers the need for high-octane gas), but they also reduce power output significantly for the same reason. Worse, a dished piston prevents your combustion chamber’s quench area from working properly, which actually increases octane sensitivity. You are much better off with a properly sized combustion chamber, running flat-tops (with valve reliefs if you need 'em!) for better flame propagation and improved quench. On some engines, like the open-chamber Clevelands, there’s not an effective quench area anyway, so you’re kind of hosed no matter what you do. Invest in octane booster, I guess.
