yup, the 120/80x18 will work on the rear. I have the BT45s on my Seca550 race bike. I ran them at Grattan last summer. The front worked good and the rea was a little funny. I think it is because I have a 130x18 on a WM-3 rim and it makes the profile funny. I am putting a wider rear wheel on this year and try the 130 again. The font stuck and the rear ould slide in turn 2 and 3 and 4. I also had some ground clearance issues and bad shocks so I am not willing to blame it on the tire. Gordon Hamilton runs them on his SR500 and he can win races.
Ken
AHRMA 412
Vintage racing - old guys on old bikes