LSx 1967 Mustang

[Hornet LSX]

Just out of curiosity what is the measurement between the shock towers? The reason I'm asking is AMC cars share a similar suspension and always looking at exhaust options to make LS motor work in AMCs. Hornets in particular. I'm thinking you might be onto something with the 351 headers with LS flanges added. I realize it's a moot point now but did you mount the LS flange to a strong flat surface? I have seen the warping problem in other project post and the solution was to strap the flange to a thick flat bar stock or spare cylinder head to keep it from warping when welding. Just a though if you decide to salvage the 351 headers for your build.
Concerning the LS1 VS LS6 intake. The advantage is from 5,500 Rpm + for added HP. Long tube exhaust over LS manifolds will potentially move the torque to lower RPM levels. So some gain in power in the lower RPMs will be a little gain in MPG potential.
Looks like you are getting things worked out. Enjoying following along with your progress.