I have a 68 that I converted to electric wipers. I bought a complete set up from a later Bronco - linkage, motor, wiring. I installed the arms/pivots it in the factory holes in the windshield frame. I installed the wiper motor in the stock location for a 69 and up Bronco so the right wiper was in the stock location. Then I extended the linkage over to the driver's side arm. I did this with a round rod I clamped in place then cycled the wiper motor (by powering it by tapping to 12 v so I could stop/start the motor). Once I had it close I tack welded it in place. It was pretty close but I ended up cutting the rod and threading the ends then connecting them with a turnbuckle so I could adjust it to make it perfect. Works much better than vacuum!
I did the Cherokee wiper arm mod but that was rubbish. Just modify your existing arms to hold a 9" blade.