I registered to reply to this, great forum BTW.
I completed a 5.3 swap into my 1981 GMC K2500 about a month ago. I used Hooker adapter plates from Speedway motors. You can buy them with different engine offsets or stock. I used the stock location and it worked great. Hooker sends the plates with all the needed hardware.
I looked at pre-made swap harnesses, and I would recommend that you just do your own. I spent about 1.5 hours in the garage modifying the stock GM harness to suit my needs. The only thing that tripped me up was the VSS signal in and out. One comes from the trans and one goes to the instrument cluster (in the stock configuration). I had it wired backwards and the engine didn't know it was moving, leading to some drivability problems when I pushed in the clutch.
There are a couple of reason I would recommend you modify a stock harness:
- you will end up doing the cutting or adding to a painless, or other aftermarket harness, when you want to add or delete something in the engine harness IE: 3 wire temp sensor for your stock gauges
- the stock harness is built with very high quality wire and is a fraction of the cost of the Painless harness
- stock harness already has the shape of the engine and the split loom installed and sized correctly
Once you remove what you don't need, it becomes a 3 wire hook-up to make it run. That's a little misleading, as you'll need to wire in all your gauges, cruise stuff(maybe), VSS, and a couple more things I'm forgetting. It will run when you supply ground, constant +12v and ignition switched (while cranking, HEI wire works well for this) +12v.
I ditched the A/C, so I can't help you with that. I had planned on reworking the frame in that area if I were to keep it. It's cheaper, in my minds eye, to modify the frame rather than buy an aftermarket setup. I think it would also be pretty easy to build a mount for the A/C that moved it up and away from the frame. The A/C on my engine had it's own belt/tensioner so it should be easier to build something that would work well.
Just some random thoughts, hope it helps.