Holley has made a dozen 'Model' 4150/60 carbs many of them marketed as '750s'
a. To know more about the carb on hand - look for the 'list number' and then look it up on
Holley's Carb List.
If the carb has a choke tower its stamped there. It may or may not also be on a metering block.
Accelerating from a dead stop involves almost all, if not all of the circuits and adjustments in the carb.
The main jets are mostly to control high speed cruise fuel mixture. To be more accurate, the main circuits contribute fuel when the air velocity past the boosters is sufficient to create pressure differential that pulls fuel. IF the system is calibrated properly, this will typically occur on flat ground steady conditions (no trailer, etc) above 45 mph, and be dominating the fuel contribution by 60 to 65 mph. With a new fangled carb with additional emulsion holes and weird air bleed combinations etc, then those have to be fixed first.
When looking at a spark plug, one has to know something about the conditions it was subject to.
If that plug was a full wide open throttle run in top gear, the porcelain looks lean - but's just the tip - and there's a ring of carbon on the first thread.
If that's a fresh plug with mostly cruising and a couple of accelerations to 60 mph, maybe its a bit lean under some conditions. Closest to the gap is where the flame is converting from an electricity supported kernal to a self sustaining flame front.
Anyway. That's a long way of getting around to this.
If you've driven it on the highway without problems, its not too lean in the primary main jets.
Go drive it on the interstate like a normal driver. No wide open throttle. Add moderate throttle as needed for hills, normal passing, etc.
If it is jetted too lean the engine will rev and die or nearly die. Slow down, take it home and go up a size or two primary main jets.
If you want a decent info from Holley find yourself a used copy of
Holley Carburetors and Manifolds by Bill Fisher and Mike Urich, any edition.
You're buying for a good intro into how carbs work and the diagrams, not the marketing content.
Alternatively for less, the smaller
Holley Carburetor Handbook, Models 4150 & 4160, Selection, Tuning & Repair By Mike Urich
The sequence of tuning does matter. Sometimes we have to go out of order, but it then means some more iterations of testing.
Remember to start by checking the fuel level in the bowls.
Then
Tune for Performance