What is a good crossflow muffler that will go in the stock location? I am running dual flowmasters
and looking to go with a good single crossflow with a good sound and performance. How about a complete system for a big block?

Check out Dr Gas, they have a website I think it is drgas.com. They are know to make excellent "x" pipe systems.

The only "performance" type dual in/dual out crossflow I know of is made by flowmaster. I have this system on my nova (force II system) and it sounds good, with a lot more ground clearance than dual mufflers.

I second the Force II system. Sounds good, no resonance in the interior. I plan on adding a X-pipe soon.