A little knowledge is a danerous thing.
At the very least install an electrical solenoid to turn the whole system off and on. You only need propane at WOT. You could wire it to the kickdown switch, at least that way it would flow gas only at WOT. Also install a system shutoff for the driver, so he can deactivate the system at will.
Do I even need to address the obvious safety issues with your present, hand-controlled system? ( driver distraction, loose fuel rolling around in the car, no control in the event of an accident....)
You need to do a little research to determine the proper size orifice. Orifice size, combined with the pressure in the system will determine propane flow. The sound you hear when you add propane at idle is similar to preignition in a gas engine--it will destroy the engine. When you hear that, you have too much propane.
Using the BBQ grille regulator, you must add propane before the turbo. The BBQ regulator is set at 11" water column or a little under 1/2 psi. You can inject propane at 0.5 psi into a turbo-charged air stream with a pressure higher than 0.5 psi. Adding it pre-turbo also promotes a more uniform mixture.

You have demonstrated that propane works, now add the required system to make it work safely and not be detrimental to engine longevity.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags