IF you choose the Edelbrock carb, it is IMO you will like the Thunder over the AFB. Seriously consider this.
On the Summit carb, I have had the Holley version before Summit picked it up. Worked well enough I suppose. I had the small spread bore version. IMO, I thought it lack luster.
I also suggest a 2-1/4 inch pipe, NOT a 2 inch pipe for your dual exhaust. The engine can use it.
Muffler wise it is hard to decide on on what will be a quite enough muffler for anybody since it is so very objective. Though a good sounding muffler that is quite at cruise with exhaust manifolds that I have personally used and would do so again in a heart beat flat was the Thrush Turbo mufflers. I used them @ 2-1/4 off exhaust manifolds on a 360 engine. Now do keep in mind that the car had Hwy. gears @ 2.76 and a 26 inch tire.
What does that mean? It means low RPM's at Hwy. speeds that also translates into an engine spinning easy and not jamming the exhaust note out. We all know it's quitter at cruise than wide open throttle and 90+ mph.