Hi Brian, the measurements you give are about right compared to this one I found.
The blade is about 7" long.
I don't know whether the later tensioner would work (I'll take David's word for it), but given your measurements and mine, the one you have should fit.
There may be some trick to assembly that we are unaware of?
It looks to me that you might be able to fit the blade, then pull up the spring to fit the pin.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.