[JGray]

I just finished dialing in that same carb on 71 Blazer/ZZ4 combo - my elevation is similar to yours (a little below 4000'). I ran that carb for a while as is with no tuning and had a stumble on hard acceleration. I talked to an Edelbrock tech who concurred the 16 deg. base timing I ended up at was a good place to be at this altitude. He also seemed pretty adamant that my stumble could be addressed by adjusting the secondary air valve.

After numerous iterations, I ended up richer on the primary rods (65x42), staging rich 1 step sooner on the step up springs (pink - 7"?), moving the pump rod linkage to the outer hole and bending the linkage slightly to maximize the shot. All that helped, but it took adjusting the secondary air valve per the tech's recommendation to really make everything happy. I did end up taking the air valve adjustment past where the manual says to stop turning the screw (delaying opening of the air valve).

I'm very pleased with how it runs...

Edit to add - just noticed your 6 deg. initial timing and that is where I'd look first. That sounds low for your altitude - my ZZ4 spec is 10 deg. initial and 16 is better at similar altitudes. With a similar engine and altitude, mine responded well to a bit more fuel.