I have much the same engine configuration in my T-Bucket. 350 SBC, 1406, RPM Air Gap, 487 heads, 9.1 cr, pretty much everything else is stock except for MSD ignition. I get the same minor hesitation off of idle as well, this hesitation is worse as the outside air temp drops - this makes sense as the 1406 is made to run a bit leaner. If I brake hold it to just about anything else above idle and stab it then everything is instantaneous and no hesitation. Any part-throttle stab is smooth. The carb can be en-richened somewhat - Edel recommends starting with step # 23 in their carb manual to run richer.
I called Edelbrock about this a while ago. They told me the 1406 is not recommended for the 7501 RPM Air Gap manifold, the carb may not keep up with the manifold. They indicated that a 1405 would be a better match for the manifold or a Performer Air Gap (among other Edels) would be a better fit for the 1406. Since then I have seen the same advice elsewhere. The RPM Air Gap is made for performance from 1500-6500 RPM while the Performer Air Gap id made for performance in the idle to 5500 RPM range. The RPM Air Gap's performance envelope starting at 1500 RPM may explain the hesitation off of idle.
I went through all the other Edel suggestions just to be sure there wasn't something amiss - accelerator pump adjustment and function, shafts, vacuum leaks, idle mixture. While testing, I found mine has less hesitation on full-time manifold vacuum rather than ported vacuum. Timing is 12 BTDC and all in about 2600 RPM, pulls 21" steady vacuum with a silky smooth low idle.
I was considering changing out to another manifold last time I had the heads off, but put the 7501 back on. This is a street cruiser and the street manners of this engine are just too darn good to start messing with it. It may not be the combo Edel would recommend, but it works very well for me. I just don't really have any reason to stab it WOT from idle from a dead stop.