alan, thats pretty much the same thing i want to do down the road.
im sure you could just have the magnaflow system bent upward to hug the floor pans to get that x-brace to fit. the only problem with that is your toesies might get a little warm. do you have the 2 1/2 or 3 inch system?
yeah, the addition of the mante carlo bar is a must. it made a world of difference on my fathers 64 1/2.

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66 coupe. 5.0 block. edel rpm heads, z-303 cam, crane rr, pro flo efi, lt headers. t5 and currie 9 inch, tcp coils, 5 leaf rev eye, caltracks, watts link, and 4pt. rollbar. next: subframes, & slicks!