Carb tuning, larger accelerator pump nozzle?
From Dano's description I think the problem is before he gets the throttle even that far open.
@dano You killing me. Spill the beans on what you're doing when this dead spot occurs.
My cousin had an old truck that could be put into without any throttle! I'm betting with your setup its a little different. But this whole question about what circuits are in play can be narrowed down if we have a sense of how much throttle, rpm, and load (gear).
Its killing me too, this ain't my first rodeo and have never had an issue like this. The 460 I build with a manual trans and a box stock 3310 Holley I could just let the clutch out and roll and 1st wasn't a granny gear either, 33" tires and 4:10 gears in a 5500lbs 4x4. Have built and tuned several engines with my dad, 410 Merc, his 440 in the Coronet (which has the next step up cam than mine and a 4 speed/3.91:1 gears, he has a 870 Avenger on that), 2300 Ford with a 390 Holley, several 302's, 350's, 360's for friends.
I take that back, I've had this issue once, with a used Edelbrock 600cfm AFB that came with the engine, it would bog off the clutch in my truck with a 351m and 4 speed. Tried to tune it out but ended up replacing it with a 1850 Holley and the bog went away. I chalked that up to worn throttle base plate as the idle would change if you sprayed a water mist near them.
I expected some tuning with this carburetor.
So when stopped, in 1st gear coming off the clutch and applying throttle it dips lean and you can feel the bog, then it catches up. Classic scenario you see all over the net, "X brand carb bogs off idle". Just letting off the clutch slowly on a flat surface it will bog and die with 750 or 900 rpm idle. No power brakes, so no booster leak (been to that dance). Were talking driving around the neighbor hood, going over speed bumps, all the
"daily street driving" things. Top speed of maybe 50mph but you can see and feel the lean in between gears.