My boat is a few years older then yours, being a 1980, but I
don't believe that the shifter cable design changed in the 80's,
unless your shifter is mounted on the wheel pedestal.
My shifter is mounted to the port cockpit bulkhead, near the sole.
My shifter also bound up at the beginning of the season last year.
I simply lubricated well at the transmission side & at the shifter mount.
After lubricating with a combination of WD 40 & PB Blaster it freed up the
cable & worked fine. I believe that the trans. shifter gets jammed where the shifter handle bracket meets up with the shifter mount, as there are notches in the two round plates that meet to lock the shifter in forward, neutral & reverse notches. Access it from the inside of the port cockpit storage locker.
By the way, PB Blaster works well to free up corroded metal parts, but I'm always careful not to spray it on anything with rubber seals, as it is very corrosive to rubber & plastic.
ps. After this happened I always make sure that my transmission shifts from forward to reverse before I release my dock lines. Its pretty disconcerting to release your lines & start drifting out from the slip, only to find out that the engine won't shift into gear!