66' Barracuda and 273 with headers

Perhaps use the factory as guidance. Did they ever ship a car w/ headers, even as a 273 race car? The HiPo exhaust was special, but still 1-tube. I read that they tested a dual-exhaust and found it gave slightly less max power than a single, or perhaps a worse torque curve. Good they talked to you. They would likely shun a slant-six, as the bad-teeth guy diss'ed one on a cable show (Horsepower Nation or such), though a youtube shows one running a 9.09 sec 1/4-mile after a turbocharger and other mods. For sure upgrade the points ignition. A 1985-95 Chevy 8-pin HEI module would be good, especially if your FI Tech can command spark advance (old Holley Commander 950 controller can) so no fooling with advance weights and such.