Brian Sturdivant might have been interesting--at least he had beautiful bone structure and would have looked tasty in hippy garb!
Oh, Gothick! You
do have a way with words!
El Sturdivant also appears in Von Richthofen and Brown, which has been released on DVD and is available from Netflix. According to IMDB.com, he plays a character named 'May'.
As for Ray Carlson, I have absolutely no recollection of him or his character, Aldon Wicks. Anyone have a screen cap on hand?
I agree with MB that Tom Happer might have worked in the role.
I, too, wish the scene in the boutique had been filmed, as it would have likely been shot on Tarrytown's Main Street, with which I am quite familiar. It would have been fun to see what the street looked like back in 1971. I wonder if there was a suitable boutique already on the street that they had in mind to use. And, yes, either Prentice or Stroka would have worked as 'Gregory'.