EFI engines can run on CNG without major changes. The big expense is for high-pressure tanks, regulators and fittings, since CNG is dispensed at up to 3600psi. This differs drastically from LPG systems.
My brother has owned a CNG (factory) Crown Vic. Now he has a CNG Honda Civic. Both engines looked no different than the gasoline versions. CNG runs so clean that the tailpipe has no soot at all. It looks like the engine was never started.
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.