This may sound a little dumb but, did you buy new mounting screws for the linkage? If so, did you trim them down? I put my linkage on my 69 and had the same thing happen. Turned out to be the new screws that I bought from Ricks and I never trimmed them. Hence, they were restricting the linkage/wiper motor from traveling through it's full rotation. Just a thought.
1969, X-44, Export Model, Hugger Orange, Std. Int., BC Rear(soon to be a BT 12 bolt), PDB, GMPP 350HO, M22 Supercase, SSM Lift Bars, AirGap, Holley 750, FlowMaster American Thunder Exhaust, FlowTech Headers