You must be one of those young guys to call 1986 "circa." Makes me feel old.
Anyway, what you are going through is what I'm going through right now on a 75 engine I'm rebuilding. I have pulleys and brackets I pulled off my 77 engine but for some reason I can't get them the pulleys to line up exactly right. I think what you will wind up having to do is to decide what you want to end up with -early engine or later engine - and convert everything over.
In my case, I have rams horns from the 75 and will be converting or trying to convert everything over to long water pump. Problem is finding a complete engine to take everything off of it.
Also in your case, since you're not going to be using the truck for putting around town and only an occasional trip I wouldn't worry too much about manifold size. I doubt it will have a noticeable effect on performance.