71' 340 air cleaner, choking out my little 318. WHY??

Combo:73 swinger, 69 stock bore 318, 714 casting heads with stock size valves, summit 6900 cam( close to stock 340) 9.59:1 compression, 800 thermo quad, good flowing manifolds, 2-1/2 exhaust, o/d 4spd, 3.91 gear,27" tire, twin scoop functional hood. Timing 17* initial 32 total, all in by 2600.
Problem: Wind out 1st gear at approx. 4000 rpm( foot to the floor)she noses over- shift, she winds to 3500-4000, noses over shift maybe 4500, noses over in 3rd.
Attempts to fix: Took out k&n filter bolted lid back on/ no change. Then took off wing nut, thinking the pressure was somehow messing with the metering rod bridge or the air door/no change. Then took the lid off, and she pulled strong to 5500-5600.
Thoughts: There is no way this mill is hungrier than the car it came on from the factory. And even if it is, that air cleaner should support a high revving big block. Is it on a fine line of being rich in the secondaries, and no lid leans it out enough? It only has a 110 secondary jet.
I WELCOME ANY AND ALL QUESTIONS, COMMENTS, AND/OR CRITICISM. Thanks in advance