OK all, to clear up a few things here... first of all NO pre-67 Cuda came with dual exhaust, period!!! The trans crossmember is only cut out on the drivers side to allow the exhaust pipe to pass through. Next, the Formula "S" exhaust system had a transverse mounted resonator and no muffler, but came out only on the drivers side in the back through what Scott now knows is a dual cutout sized rectangular hole. I have seen an NOS setup laying on my buddies floor for what now is going on 20 years it seems!!
IMO, it's going to be a difficult find to locate a used valance that is cheap enough to cut up for the dual exhaust cutout conversion, and the repopped ones are much too expensive to buy to just cut up, for most of us anyway.
And remember Scott, you still need to modify your trans crossmember to allow the passenger side pipe to pass through the crossmember area, going under is pretty restrictive and hangs too low!!! Need a pic of that conversion?? Message me and I'll send you one. I had about a dozen crossmembers converted a while back and sold them all to members here for their conversions!!! Geof