Probably not your issue but have you checked to see if the center nuts on front shocks are torqued properly? There were a few threads with that problem. You also said the car has headers, have you checked the steering shaft area where it goes into the rack, when you install headers on these cars you have to remove that steering shaft. Maybe front sway bar bushings? Subframe bolts all good and tight? Just throwing random crap at you to look at. You probably have as you seem competent with a wrench. Hope you get her back on the road soon