My headers nightmare

Yup, sounds like a new shop needed to me. It shouldn't take a week to figure out they won't fit, and if they didn't get them to fit they sure as heck don't deserve $700.

I have Doug's D453's on the 340 in my Duster with Summit's 2.5" dual exhaust kit. Worked great. And I've got a 4 speed and all the mechanical linkage to deal with too.