A standard ST would be 73* as you know, the TT is on the Longstaff is 53cm?
76 degree** mean that the top tube on the Longstaff is realistically 56cm, so I reckon a 57cm would fit you very well without looking all T-rexy and needing to buy new component to make it fit again***.
*with normal saddle setback, or even inline with the SAA saddle.
**1 degree steeper = 1cm longer give or take.
***I tell a lie, you probably need a 10mm shorter stem.
A standard ST would be 73* as you know, the TT is on the Longstaff is 53cm?
76 degree** mean that the top tube on the Longstaff is realistically 56cm, so I reckon a 57cm would fit you very well without looking all T-rexy and needing to buy new component to make it fit again***.
*with normal saddle setback, or even inline with the SAA saddle.
**1 degree steeper = 1cm longer give or take.
***I tell a lie, you probably need a 10mm shorter stem.