When I swapped from a 904 to a 727 in my 73 scamp I was getting a vibration above 40 mph. I noticed that the 727 didnt have the counter weight on the tailshaft and the 904 did. The weight off a 904 wont work on a 727 so I had to look in 3 junkyards untill I found one, no more vibration.
Just something you might wanna look into.