The three screws that retain a washer motor haven't changed since 1958 and just about any wiper motor you find (whether using the square motor or the round one) will bolt up. What is different is the linkage that bolts up to the ball on the wiper motor arm (which you should have) and the washer pump and park function. On older motors it was cam activated (mechanical) after 1985 it became computer controlled as pulse wipers where introduced.
Any mechanical parts that you can not find for your car exists as a reproduction part on Corvette reproduction houses web sites. A warning however as Corvette owners do have deep pockets so the price for the same GM part is higher for a Corvette (just be thankful you don't own a Cadillac as the same part that fits a Chevy Impala has a different part number with an seriously inflated price on it because "Best of all it's a Cadillac" even though GM used the same part on all of it's B-body cars) Same thing; a Corvette is a just Chevy Impala frame cut down to eliminate the rear seat with a plastic body stuck on top. The Corvette uses a lot of Impala parts and light truck parts to make a high speed sports car out of the family sedan (at least back in the sixties).
Parts interchangeability is a lot easier with a GM product than with a Ford or Chrysler product. Your Ford parts counter man will ask you not only what year your car is but what day of the year was it made as Ford's "Better Idea!" was to change the design using different non-interchangeable parts on the same car during the same production run.
You might want to buy an old used copy of a Hollander Interchange Manual that covers your year car (they published a new manual every year, but each manual covered five to seven previous years of cars, so you want one published about three or four years after your car was made to get a larger list of possible donor cars).