Slingshot rear shock or. What am I doing wrong

[dirky]

Hi

Need help figuring this out. 

Frame as far as I know is a 89. 750. So are the arm and linkage.  Mocking up with a  bandit shock.  320mm eye to eye centre. 

It’s pushing the linkage right down so much too big but when I set the arm at a better angle and put the tape across it I’m getting  290  I thought the standard shock was 319.  

Can anyone see where I’m going wrong. Cheers

Edited February 12, 2021 by dirky
Measuring error

[dirky]

Might be sorted. Found a 290 lurking around