A lean condition can cause pinging. It's possible those #1406's were set up too lean.

The carburetor cfm required for a street engine is calculated by the following formula:

cfm = ((engine cid x max rpm)/3456)* volumetric efficiency

((455 x 6000)/3456)* 0.80 = 632 cfm
((350 x 5000)/3456)* 0.80 = 405 cfm

The cool thing about quadrajets is that they have vaccum secondaries. Even though they typically are rated over 700 cfm, the engine only uses what it needs. If there isn't enough of a pressure drop to open the air valve - it doesn't open.

I've got a Q-jet on a 350 with the 2bbl heads and cam and a 2.78:1 gear. The secondaries aren't quick to open. When they do, I love the waaaah!