Noob with a new mx5 Saturn build.

[garyt]

just adjust the height of the cross bar to suit the belt slots in the seats and position the threaded bosses to suit the width of the slots. in the book it says "adjust measurements to suit seats"
if the cross bar is to low, the seats become load bearing as the belts would come upwards through the slot pivoting on the slot edge to your shoulders and down.
I know some people have had to provide seat structure reports for iva

a bit long winded but as said above the position of the crossmember and bosses was intended to be "to suit" rather than set in stone
hope this helps gary