I installed tti headers on my 66 Polara. Only issues I ran into were:
idler arm hits a pass side tube,
had to use shims for the engine mount so pass side would clear said idler arm.
I can't have a grease zert in the arm or it's worse.
I know my car was in some incident, my stub frame is beat up so I'm not sure how straight it is, in any direction. I used an aftermarket idler, so a factory piece, or Mopar piece might not interfere. And finally, my motor mounts are probably stretched out with the way I drive, hard acceleration whenever possible. They are close to 10 years old.
I still love them, I had normal stock manifolds and would have gone with other exhaust if they had 3" available, but it wasn't and I had money to burn and I'd do it again anytime. Cheap they are not. And when I checked the bolts after driving 50 miles tti recommends, they were perfect, didn't back out. Save your pennies and buy a set!