[jhv48]

I actually have both of the aforementioned carbs. I have a Prosystems 850 on my Roush 427SR and a Quickfuel 850 on my Lykins Motorsports 427. Both are easily tuneable and will wake up that engine. The stock street avenger is the first thing I would replace. Cost is around $750.
The sidepipes are the second thing I would replace.
Full Throttle in Sparks Nevada will modify your pipes but they will be loud. Mine cost $1200. Basically, he sent me a pair that had already been modified and I sent him my used pipes plus $1200.
There are other side pipe manufacturers out there that will do a fine job of making a set of pipes for your car. Do a search and you'll find lots of recommendations.
Pipes and a dp carb will get the air in and out so you can use every horsepuppy your engine can churn out.