Any aftermarket or under dash air will cool that small space. (very small cu ft) You'll get the usual 30 to 50 deg differential from outside. Depending of course on how hot the outside is and humidity. 134 is the one you are supposed to use as you know. Only person that can buy R12 are "Technicians certified to service motor vehicle air-conditioners by a Section 609 EPA-certified testing organization". You don't worry about BTU's in a truck cab or any other truck/car. Proper sized radiator will suffice to accommodate A/C. It's old technology and all worked out already. As you know, the more insulation the better, including all air gaps, I.E. rubber seals at doors, tinted windows, But you will be cool without that also because it blows right on you, it's only 30" away. Build your own, Get a Sanden compressor, a drier, a condenser, evaporator, a under dash unit and you got it. There are many old school under dash units available on EBAY and Craigslist. They have the evaporator and blower built in just like the "new" ones. Some are thin and it would be in the middle where no one sits.
