Take it to an automotive AC shop and have them recover the refrigerant. They should be glad to do it if it has R12 in it.

Yes.

Use the AC bracket from the original engine, and any pulleys etc. that are necessary.
If you decide to keep the AC you might be able to unbolt the compressor from the engine and replace the engine without opening any refrigeration line.

If your going to remove the lines YES. BUT you can more than likely
unbolt the compressor and move it out of the way enough to get the
engine out.

As long as it is the same size engine with the same electronics.

Yes, unbolt the needed brackets from the engine your pulling and mount
them on the replacement engine. Then bolt it back together.

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