Hiya Boxhead.....I'm using the two blade wiper system..the 3 blade will be different.
My motor gear is 32 tooth 120 degree sweep. The pic you show is 32 tooth 105 degree. so you have to fiddle with the sweep or go find a wrecked BMC car and check the gear. Mine I think is from a Marina.
If you dont wish to look for a Marina, then modify the gear wheel as per this site:
http://www.erareplicas.com/427man/wiper/index.htm
There are , as suggested, also slight differences in the diameters of the gears on the wiper boxes which can have an influence.
Use trico A103 arms which you can trim to length. The length of the trico arms is governed by the vertical distance from the center of the wiper box shaft to the top of the screen glass. This ensures you wont wipe over the chrome screen surround at the top. These trico bits will polish up to a chrome like finish if you want.
I park mine on the right hand side and the blade lays against the bottom screen base and extends to about 25mm inside the screen side upright. It then wipes to 120 degrees to the left (the blade bottom almost overrides the chrome screen botton at this point). The left hand blade overwipes that part of the arc to 120 degrees to the far left of the screen.
Wiper blades are available in 10", 11"and 12". I'm using 11"which I had to fiddle with and trim a bit to use but they work well without overwiping any of the screen surround or seal.
I believe the wiper gear you have is the one used on the 3 wiper system.
Cheers mate