Problem: WHICH Street Avenger?

There are four different sizes.

The biggest problem seems to be with the 670. My 570 worked very well and was better than the List 1850 600cfm Holley it replaced.

The 670 is supposed to be pretty lean and swapping a 750 metering block into it should help.

Those plugs don't look like you have a lean condition to me, but surging at steady highway speeds sounds like it's lean there. There are a lot of questions. For example, idle mixture screws usually are set up to baseline at 1 1/2 turns out from lightly seated. As you get higher in altitude, they tend to go in a little bit, like maybe 1 1/4 turns out from lightly seated. But, being able to run at 1/2 turn tells me that you're getting fuel from somewhere else.

This is going to be a long process and you need to get a good book, as mentioned above, then tackle one problem at a time. I'm with RR on the first thing to do being buy a real Holley carb kit, take the thing apart and make sure everything is clean and sparkly. Idle and high-speed air bleeds need to be open, float levels must be set properly, power valve checked for integrity (even though all Street Avengers have PV protection).
There are probably a number of things that are contributing to the problem.

It'd be nice to know what your cruise vacuum reading is.

And what is the List number of your carb?

R.