janrein - my Stinson has 3-point harnesses in the front seats; the original 1947 manual just says "each seat is equipped with a safety belt" but it's not possible to tell what form that originally took.
So I don't know what was original, but it's certainly possible to fit 3-point harnesses. Ours I'd say are at-least 20 years old.
At risk of stating the blindingly obvious, the airworthiness standards for light aeroplanes for at-least the last 30 years have mandated upper torso restraints in the front seat. That wasn't done for decoration - they're there because they improve safety (crashworthiness if you like).
G