Due to the age of the furnace, I do not have the manual available, but I an tell you that the fire box has a thermostat controlled switch, that turns the fan on when the fire box temperature gets to a certain temperature.
This plenum thermostat, does the same function as an on and off toggle switch except it is temperature controlled.
Warning: Do not attempt this unless you have some basic electric knowledge. The voltages inside your furnace can cause sever shock and or death. Only open the furnace with the power turned off.
This plenum thermostat is normally enclosed in a sheet metal box. When you remove the box's cover to gain access, you will see a calibration knob.
Do not touch this control. It is factory set and could cause your furnace to get to hot if adjusted incorrectly.
Turning it could also cause your furnace not to work unless the thermostat switch was replaced.
If you look closely you will see 2 power wires going to the plenum thermostat and another brass looking tube going to the fire box, or the box it's self can be in front of the box with a large tube going into the fire box.
One of these wires will show hot and one will show no voltage. Put a toggle switch to allow it to connect these two wires and your fan will run.
Turn the toggle switch off and the plenum thermostat will control the fan.
These instructions are generic in nature and may or may not describe your furnace. I believe I have supplied enough information for you to do this modification to your furnace
More questions? Just ask.