Well if that old engine the guy has was in that truck before it be pretty easy to install bec those old engines were pretty self contained so not much to mount just the engine mounts and transmission and exhaust.Then your basics like gas lines or brake lines rad lines etc.I'd make sure I had some books , diagrams , full engine host etc and it be nice to have an extra set of hands that have some engine or mechanical experience too.
Again if that old engine is running good those are pretty good old engines and one of the best V8's ever made.So to me really if one of those engine has low mileage that be the one I'd use but thats me.
You would really want to make sure the engine you decide to install has been tested or is known to have previously run well.I'd not really wish to install it unless I knew that the seals and gaskets , timing was good, oil changed , engine flushed , rad flushed etc.Whats great about those is they have lots of clearance under the hood and space compared to many of the much newer designs so it's easier to work on then most.
Hope that helps and best of luck.By the way if you've done other engine work like this I'd not be too worried about it bec it's basically straight forward.