Before I would just replace the complete truck wiring which is a major feat to accomplish since there are NO GMC specific harnesses available I would start with only the ignition circuit and see if you can get the truck to start every time you try. Just wire up a simple stand-a-alone harness to get it going and gradually go from there - You are already in a bad place cause the original motor/transmission has been replaced with a off breed donor engine - wonder why they used that motor/transmission combo but I guess they had it sitting around and said "Why not". Start with little accomplishments and once you gain a little experience and feel warm and fuzzy about it then go into tackling the prior owners cluster of a mess. I have seen many project cars and trucks abandoned and left for scrap because the new owner didn't have the knowledge, mental drive or the funds to finish. You can do it! Ever heard the old saying - "How do you eat an elephant? ----- one bite at a time."
