This might help...
I have a '72 GT, the heater may be the same.
Turn thermostat off.
On the front of the heater remove the panel, if there. (Mine is gone, working on replacement)
Behind this panel is the pilot light - unscrew the glass cover found there. This hole is where the pilot is lit. To the right of the pilot hole is a knob.
Push the knob in and hold, light pilot light, keep holding knob in for about 30 seconds. When you release the knob the pilot should stay lit.
Replace the pilot cover, now turn thermostat up.
If you don't put the pilot cover back on, the pilot will blow out when you turn the thermostat up.
Hope this helps.