I have not used the street avenger carb, but I do have a 670 truck avenger carb and the transition circuit was lean. I think the idle restrictions were 0.025" or smaller. I drilled them to 0.028" and it really helped fix the off-idle stumble, and I was also able to get more adjustment from the idle mixture screws. Before, I had the mixture screws about 3-turns out and the mixture screws has very little effect. After drilling the restriction, I was able to set the idle mixture screws at 1-1/2 turns.
I think the colder plugs would make it easier to read the plugs. I have been using the NGK V-Power R5672A-8 spark plugs in my Brodix B1-B/S heads for years with no problems. I am also running them in the new engine with the Edelbrock Victor heads, but I have not looked at the plugs in that engine yet. I am not sure why replacing your plugs helps? If the plug gets too hot it can cause pre-ignition. I mean really, if you have to replace the spark plugs anyhow, why not try a colder heat range?