RE: manifold heater O-rings replacement
The nozzle is pressure sensitive and should remain OFF for fuel pressures below about 60 psi, which normally exists on the fuel feed line from the IP. The flame heater pump produces 90 psi, which opens this valve. Original intake heater systems used shut-off solenoids in both the feed and return lines to make sure that no fuel leaked into the manifold. For proper operation, the nozzle must produce a spray pattern of atomized fuel, I think a 60 degree wide pattern is okay.
You can test the nozzle with compressed air by varying the pressure and observe the action. Any O-ring that is not effected by the fuel should be suitable (see Devin's post above).
For more, see Military Vehicles Mag article, issue 99, Oct 2003.