You will probably need to get it certified and any shop will likely not certify it without taking the tanks out and and re-valving them and pressure testing.
That all depends on whether or not you can find a place that will fill it without checking for a cert sticker, if you can get around that then you can do it yourself if that is your intention. In that case, to answer your questions; cost depends on how expensive the parts you buy are and really, the only decision on the actual conversion would be placement of the tank/s (in bed or under) and whether you want to keep it on gas or not, the rest is pretty cut-and-dry; Install tank/s, hoses, lockoff, evaporator, mixer, hose from evaporator to mixer, hoses from mixer to throttle body, hook up sending unit. * Install alternate fuel switch if you are going dual fuel... You'll also need to find a way to kill the injectors if you are going to run dual.
I have a '97 F250HD 7.5L I did some extensive checking last year and found it to be about $2500.00 depending on the tank size. There used to be some tax incentives for the conversion but they might have expired in 2004. I have renewed interest in doing the conversion again since gasoline is climbing this season. Also I was lead to believe that in our fuel injected motors, the LPG would be supplied to the motor via the existing injectors and not via the air intake as in carb engines. Please post what you find.
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