[jhv48]

Replacing the carb with a mechanical secondary Holley is a great idea even if you can get the Street Avenger to perform properly. Vacuum secondary carbs are great for heavier cars but not ours. Even Holley recommends a mechanical secondary carb for cars as light as ours.
I, too, had a roush engine with that street avenger carb. Car performed ok, but when I replaced it with a mechanical secondary carb, it was like a light switch was turned on. Woke that engine right up.
Good luck.