If your truck sits alot or is stored, put some cheap dryer sheets from a dollar store around your truck. Mice hate them. I even have them folded and stuffed in the vents. i left some sticking out so I can pull them out.
Factory service manuals on ebay. You list several trucks. For your 75, you would need to get the 1974 service manual and the 1975 supplement manual to be covered. The 1975 supplement will have fewer pages, but added info.
I would go with the complete Vintage Air Kit. Pretty much direct fit for chevy trucks. I have one on my Buick GS, that one was not direct fit, but it works great.