Lpg will be the easier of the two. The engine equipment is similar but CNG requires a tank to withstand hi-pressures of 2000-3000psi which are small and of limited capacity, requires an expensive compressor to fill the tank. And if I remember correctly the tank has a limited life. LPG Usally easier to get ahold of. If you want to give up the bed space you can put large tank in the bed (100 gal or so) and with an older vehicle the equipment should be easy to get. As far as legal issues it boils down to Local Authority Having Jurisdiction (Air polution board etc) as to if you can have your local mechanic do the converison or you have to go to a decaded shop.

Thats the high points I have had numerous trucks on LPG and use to do conversions in my driveway till I got older and the computers got more complicated.

I know I have not touched everthing but let me know any questions and I will try to answer them or get the answer for you

Regards