which center/drag link am I looking for?

I can't say for sure that this will help your header clearance issues, but the thing people keep forgetting is that the factory '68/'69 big block cars used different idler and pitman arms than the normal slant and small block cars- they have a greater offset and locate the centerlink slightly lower for better oil pan clearance, which may aid header clearance too. Don't know if the TTIs were designed with these in mind or not.
The big issue is that NONE of the normal suppliers (O'Reilley, NAPA, etc.) acknowledge this difference in their part listings anymore- "one size fits all". Back in the day, parts listings did recognize this and there were different part numbers for the BB A bodies. Right now, the only place I know of that lists the big block pitmans & idlers is Firm Feel. There may be others, but none that I know of other than a few NOS/Obsolete suppliers.
Firmfeel Mopar Suspension and Steering
Just so you know.
Oh, and the centerlink that you want to use with a downward-pointing stud in your idler and pitman is the '73-'76 unit.