I asked the very same question back in 2002 on the POG website:
http://pentonusa.org/forum/topic.asp?TOPIC_ID=1269&SearchTerms=78,mc5,changesAnswers here from some US gurus :
Larry Perkins :
"The 78 MC5 had 38mm Marzocchi forks instead of the 35mm on the 77. The rear shocks were more forward mounted on the 78 than the 77. There was a little more gusset to the swingarm. I think the only motor difference was a single ring piston on the 78 but I could be wrong on that. The bikes looked different. The 77 had a orange tank and the 78 had white. The frame and swingarm on the 78 were orange instead of the 77 which was grey. The forks on the 78 were orange instad of the natural colored 77 forks. The 78 forks were more rigid but the damping was not as good as the 35mm. Later model damping rods will fix this or some tech work on the damping holes. There is probably more but that is all the rememberer is bringing up. Mike Lenz can probably shed more differences light on this one. Mr. Lenz?"
Mike Lenz
"Thats pretty much it except the footpegs are lower and farther forward and I believe they may have pulled the rake in a half degree."