3 point seat belts in 68 dart

Since you asked about later model factory belts... that retractor is mounted inside and behind the quarter interior panel. Additional plastic cover at the belt exit point.
No the "reel" wouldn't be in the way but... The factory also added a welded nut and reinforcement inside the rocker structure to attach their retractor. Recreate that versus aftermarket belts like suggested above with drop loop too. Should be a easy decision.