Coolant recovery: Talk to a mechanic (or perhaps the auto shop program at a local vo-tech college) about getting it out in the approved manner. Intentionally venting R-12 to the atmosphere is illegal (it's bad for the ozone layer) besides which the stuff is getting rather dear and you might need the same amount of it later. They might be able to work with you on repressurizing the system after you're done.
Since you're taking everything so completely apart, if you're going to do an R-134a conversion you'd never have a better opportunity; though some people have their doubts about how much cooling you'll really get from what I'd guess to be a smallish (i.e., appropriate for a compact car or a truck cab) system that wasn't made for R-134a. Be prepared to blow out everything with compressed air and to just replace the receiver/dryer or accumulator, whichever you have, unless you are fully confident that the system was in really good shape and blew ice cold before the engine went.
Realistically, if it was a kid you'd be buying it a graduation present pretty soon... and again, if something is marginal, this is your best and easiest chance to fix it.
Doing the deed after R-12 capture: Transplanting the A/C system to the new engine might be easy or difficult, depending on the layout of the engine bay and accessory drives. If you're lucky, they're basically the same engine and how to do this will be obvious. You'll also be spending some time in the Jacques Cousteau position changing the heater core and control panel. You might, or might not, also need to beef up the cooling system. A good way to start would be with repair manuals for both years of truck -- look for differences between A/C and non-A/C models of 1991.
An issue irrelevant to A/C but possibly important in your future is how to get a 1991 truck smog-checked with a 1987 engine. This might be disallowed; a bit bureaucratic but technically not much trouble; or easy; or a complete non-issue, depending on the differences in emission standards between the engines -- and highly dependent on where you live.
Best of luck,
--Joe