dieselmo34,
Hi. I have almost the same setup as you do verbatim! I have a '66 shortbed stepside with a 292. Setup is an Offy 4 barrel with the 500 cfm Edelbrock 1403 (electric choke) carburetor. I have long (36") tube headers and HEI and a mild RV type cam that's a little "hotter" than stock.
My Edelbrock is a running a little rich with my setup, and I haven't changed the primary jets. I'm going to try re-gapping the plugs or maybe going with a higher performance plug and playing around with the idle mixture screws and timing.
Our engines are very similar in setup, and therefore in power too, depending on your cam choice and head work. You shouldn't have to do much more than maybe step down your primary jets and play around with your plug gapping and timing to get squared away. I don't think I'll need to step down my primaries to lean out my rich condition. If the rest of your engine is stock (especially the head and cam) you might have to.
For exhaust I have a single Flowmaster DeltaFlow muffler. It has a single 3 inch inlet and dual 2 1/4 inch outlets. I run the 2 1/4 inch pipes straight out back. Sounds great. A nice low rumble - sounds like a v8 when running at idle. If you must have dual pipes out back, I recommend a single muffler (Flowmaster, Borla or whatever you like) with a single inlet and dual outlets. With your engine, the performance won't change. Low end may actually improve with the single muffler. I wanted to get as low and deep of an exhaust note as I could without sounding like a "razzy 6" at idle. The Flowmaster was the perfect solution. I recommend a 2 1/2 inch inlet and dual 2 inch outlets. The Flowmaster I have is too large for my engine.
I personally don't care for the split six sound. I like a deep, husky but smooth exhaust note. If you don't use a crossover pipe in your exhaust system with the straight six the notes will sound lumpy like you have a burnt valve or a really radical cam. And it will be very "six cylinder-like."
Turbos are another good option. Cheaper than the flowmaster and produces great sound. I don't know if they make a single inlet/dual outlet system or not. Check Ebay.
-ftyler
