I know my opinion isn't really answering your question, but I would not do that swap. I'm sure lots of people will disagree with me, but it just seems like a lot of work for what? Another cab and fenders that will be rusting out soon? Why not just pay someone to replace your rusted stuff with brand new steel, which is arguably better outcome than the other 36 year old cab and clip.
I am kind of biased though. Hate to see another diesel chopped up for parts. Would love to see someone grab that 1985 and fix it up instead, especially for only $1,000 purchase.
Edit: besides, I also like the look of your 1976 step side. Hate to see a cool looking 70s truck turned into an 80s model.