Sears furnace model 867.763271
How old may this unit be?
Who manufactured furnaces for Sears of this vintage?
This a standing pilot unit and I am guessing at least thirty years old?

The number you posted is a Sears serial number asssigned by them. You only hope would be to contact them and , from my experience, they will not be able to help with vintage questions. They have used many different manufacturers over the years but might still be anble to get parts.