Only 79 and 80 were essentially the same bike. Some parts do cover more than that though. For example the piston is common from I think 77 till 80.
As a side question why so much effort for an old bike? If you plan on racing the vintage races then I guess that I get it. Otherwise I'm not so sure that I get it. At the end you will have a bike worth about $500 even if you spend considerably more. The suspension isn't just antiquated, it is prehistoric.