The gear box off the donor bike, a 1983 plate was actually stamped 1989 so it must have been changed out in the past. Decided to use this one and leave the 85 PD's gear box for its restoration but use its non matching motor (hope that wasn't too confusing)
Dave cleaned up and reconditioned the box which was actually in really good shape. Dropped in a 5% lower 1st gear and a 5% higher 5th. Here it is..