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Are there any differences between the mounting hardware of a '79 KZ400H and a '81 KZ440D? Will those seats interchange?

Your best bet is to look up both models on Partzilla.com and compare part numbers. The site will also tell you something like "this part was used on the following models".

In addition to checking the diagrams you can compare photos on EB.  Sometimes a part, like a seat, will have a different part number because Kawasaki made a minor change such as the seat cover pattern, but it will fit the same.