[Junket]

Yes you can only put on one balancer the outsice ring might of moved also I belive in 77-78 they changed the position of the timing marks on the balancers as well as the pointers (if they have the factory color on them Red is early & Blue is late also late is wider) to check this you might run the motor up to about 3500rpm move the distributor till it runs the best then RETARD it about 1/4" this should put your timing close then use the lite and check it aginst the factory if you are off 6-8 degrees you could have the wrong combination of timing marks.
Ken