I too went with the Flowmaster "American Thunder" setup with Hooker comp headers on my 67 convertible. I used 1" spacing in the front, 1/2" in the middle and none on the back of the brace (with grade 8 bolts). I had to rotate the mufflers a little and dent them because they were rubbing agianst the front bolts on the factory traction bars.

I am probably going to go with Dynomax mufflers to make it quieter and hopefully fit a little better, oval not rect.

Another option, JCWhitney, had a stainless steel exhaust (w/o mufflers) for $159.