Tremec TKX first cuts

I think you are correct. SSTs I believe is lower guessing 3/8 to 1/2". So I would use a flat plate with holes drilled.
The speedo cable exits the trans where the mounting flange is. So the higher the tail shaft the closer the cable gets to the inner part of the torsion bar cross member where the inner bolts are.
I set mine to the same elevation as the 833 and my cable lightly touches. I may clearance it a little more before paint.
This combined with minimizing floor work could be why SST set the trans lower.
Having an auto floor to start with