I presume you want to convert your own vehicle (1976 F250?) to CNG dual fuel operation. I believe you will need the following items for this conversion:

• dual fuel mixer (Impco Model 300?) & air cleaner
• mixer to carburetor adapter
• CNG pressure regulator
• CNG storage tank(s)
• CNG fill valve
• CNG fuel lines
• CNG pressure gauge
• electric shut-off valve for gasoline
• electric shut-off valve for CNG
• electric switches for dual fuel operation

I think that's most of it. Since the storage capacity of typical CNG tanks is equivalent to a few gallons of gasoline, ideally you should have a CNG filling station on your normal route. If not, you might also want to consider PHILL.