Did the 87 come with A/C installed but is has since been removed?
If that is the case: then yes, everything associated with the HVAC system will drop right in.
If not, ah - no it definitely won't be a drop-in change over. There are some significant differences between the Heat only/AC air handling systems and the way they are integrated with the body of the cab. The biggest issue is in the configuration/location of the firewall penetrations. It can be done (I have done this modification 3 times. I have gotten better at it, but the first time was like a circus act gone bad.)
As far as the wiring harnesses, the chassis electrical and cab electrical systems will be very similar, so you can probably use most of those harnesses.
The engine electrical is another story. Unless your 87 is a rare exception and it somehow came with a carbureted fuel system - it is almost certainly equipped with a TBI system. Given that TBI wasn't available for the light truck line until 1987 - the donor truck will have the standard 350 engine harness. It will probably be useless to you.
The non-engine stuff in the engine compartment (i.e. exterior lighting, horns, etc.) will match up well.
The transmission should be able to bolt up to your 350 - but I really don't know much about that. I tend to leave things alone as long as they are working properly.
Good luck.