That works. Keep in mind that dropping the back of the subframe allows you to wiggle the rear header into place. Get it held on and then lift the subframe, mark where the O2 will hit on the tunnel edge. Pull the header back out and make the firewall/tunnel edge move and put the header on for good.
Coming from the Bonneville world is nice because nothing was made for that car. There's no catback flanges to bolt up to.. it's all make your own, think outside of the perfect situation box and come up with solutions.
When someone says can't, my mind starts to think and look at the problem. There's an answer for every question, the challenge is finding it.
