I bought a kit from Arizona Mobil Air for my 86 B2000 a couple of years ago, the kit cost around $850 and I did all the work myself, not hard but it takes a bit of planning and it works great.
A/C units were installed in the trucks in the US ports because "final assembly" had to be in the US and that was an easy way to get past the import laws.
The OEM systems work well but assembling the used parts can take time (finding good parts as well). I'd do as 1klrton did.