1967 factory air, but blower only works on high speed, does anyone know what causes this.

If '67 is similar to the '68, maybe the blower resistor or relay. All the fan speeds except the high speed are switched through that resistor to get the different voltages to drive the fan's different speeds. The high speed of the fan is powered from a separate (in-line 30A fused) connection directly to the alternator. Or...maybe if relay is stuck in the high position that could cause it too.