Lots of questions!
The year differences are various. I have no idea where they split, there was a big difference with both the PCB and IP connector wiring from '89-90 obviously, previous to that, anybodies guess.
The easiest way to do it is simply trace the PCB on the back of the cluster. Pull it, trace out the 12V and grounds, and mark them with a sharpie, if they aren't already. (another +1 for the 90-91 clusters, already marked from GM) That makes tracing the others easier. Knock out the stuff like turn signals, lights, then head on to the gauges. They SHOULD be wired the same as another cluster you use, given a similar vintage.
You'll end up with some extra wires on the IP connector because of the diesel I expect (wait, water in fuel) but those can probably be left in place, as long as those pins don't go anywhere on the new cluster PCB.
What I did on my truck because of all the changes made between the '90 and previous, was pull the earlier cluster, make an IP connector pinout (the connector is numbered or lettered for each cavity, and also fits in the IP only one way) showing which pin fed what item, and whether it was 12V, reference, or ground, then popped ALL the pins out. This way as I added say, a turn signal, I could plug the cluster back in, test, make sure it worked, then go to the next pin. You may not need to go to this extreme, but if you notice quickly that quite a few pins changed between the two besides the ones you will simply leave in place or remove, (again: wait light, etc) I'd advise this route. In my case the later cluster used one ground and one 12V feed, the earlier used a couple for lighting alone, which made it pretty confusing. Saved a lot of time by pulling all of them, but making sure each wire was identified by color and cavity which did what. I can't emphasize enough testing BEFORE you bolt anything back up.
I second using at least the oil pressure sender and gauge from the diesel. It'll be an 80PSI unit, you need both the gauge and the sender. Gauge is an easy swap when the cluster is out. I *thought* the diesels had a different sweep to the coolant temp gauge, (240* max instead of 260*?) so you might not want to use it.
Gas gauge is different, but only for people you might let use your truck and accidentally put diesel in because the gauge says so.
Not sure if this applies, but in my state to make a diesel to gas conversion legal, you have to remove all the diesel identifying stuff, which means the gauge, and the sticker at the gas door. Just FYI, probably not an issue for you, but something to think about while you have it apart, and since you have the gas gauge already, a moot point, unless it doesn't work right.
Again unsure at what point this changed, but my '90-91 cluster has the 4wd indicator light integrated into the IP connector. Earlier (1980 for sure) there was a separate harness for this. The diesel wait light harness was also "separate" from the diesel harness I had, so you can probably unplug it somewhere close to the fuse panel and simply remove the wiring. Better than just cutting wires and leaving them. If you end up having to cut wires to get them out of the IP connector because they aren't used, make sure to seal the ends up VERY well so that they can't short to the body, and it woudln't be a bad idea to label what they were, so whoever is in there next in however many years, doesn't see it and wonder what the heck it is supposed to go to.
