I mean no dis-respect- I just don't want to see anyone get hurt. As I said earlier, when propane systems start to give trouble, I replace the entire system. There are a few challenges with just replacing 1 component at a time- the first being muliple service calls for which only 1 should have been made, now you don't know the age of which component in the fuel system,etc- the list goes on.

For a 6 cyl engine, the following peices should work fine:
Impco model VFF-30: vacuum actuated fuel shut off
Impco model Cobra:regulator/vaporizer- has 2 functions- function 1-drop propane pressure from about 300psi (full propane bottle) to about 6 psi. Function 2- change state of fuel from liquid to vapor
Impco model 125 diaphram- varies fuel flow to engine based on vacuum through throttle plate in mixer (carb).

Not sure why you have a Model L or the 2nd electrical fuel shut off. Your truck may have been a duel fuel (gasoline & propane) unit in a prior life. I have many 6 cyl engines running on the Cobra regulator with no challenges.

Also- what type of propane are you using?- Commercial, HD10 or HD5? Forklifts are designed to run on HD-5, using the lessor types does have it's drawbacks (decreased service life of propane system, emmisions, less engine output, just to name a few). If you're unsure, speak with your propane vendor. If they can't tell you, or say that all propane's the same, change vendors. I've picked up new customers with ongoing propane issues just because they were using the wrong type of fuel- it does make a difference.