For bolt in stuff, it depends a little on year of the LUV. 75 and older LUVs have different motor mount positions than 76 and newer. I really don't know of anything but the stock 75 and older 1.8L that will bolt into a 75 an older LUV. You are into fabbing mounts no matter which way you go, so you might as well go for broke and put something fun in there.
On 76 and newer LUVs, there are a couple of bolt in options besides another 1.8L. Early 80s Isuzus came with a 1.9L that is almost identical to the 1.8L other than a few more cubes inside. You could pull a turbo version of this out of an Impulse also. In the mid 80s Isuzu came up with the 2.3L 4 cyl that appears (from looking at my 86 trooper) to have the same motor mounts as the 1.8L. so it might be a good swap. I'd go with an 86 as they were carb and much easier to pipe in, but you can get an injected 2.3L or 2.6L out of newer Isuzus that should have about the same mounts, but more wiring and components to get it working. I have not yet looked close at a V6 truck or Trooper to see what they look like, but I'd be inclined to skip those and go for a Buick 231 V6 as most of what I've heard on the Isuzu six hasn't been too good.
If you are ready to mod things, just about anything can be made to fit. We've got big block Chevy motors, so if money and effort are no object pick up a 572 Chevy crate motor and have fun. V6 and small block V8 swaps are fairly easy by comparison and can be done with not too much trimming. You should expect to bring a new transmission along with anything besides except an Isuzu swap, and even then you can drop in a 5 spd so its worth doing both motor and trans.
As for instructions, look under information on the
http://www.luvtruck.com main site. There is some V8 info there, and gives you an idea of what is involved.