My Father in Law has a 38 Scarab KV, 1985 model with TRS drives. The X dimension is pretty conservative. I do not know the exact measurement, except to tell you that the outside of the outdrive cavitation plate is parallel to the bottom of the boat. The inside of the drives cavitation plate is thusly a little above the bottom of the hull.
In a more radical X dimension, your prop shaft centerline would be higher than this set up. Alot of that has to do with the bottom (step or not) transom notch or not, set back, etc. Because the model Scarab of which you speak has a straight Vee, no steps, no notch, and no standoff boxes, it is generally conservative for all around use.
He has done away with the cleavers that came with the boat and is running 3 blade Bravo 1's 23 pitch. The engines are 420 HP with some mild cam work.
Don't know about the X with the Speedmaster II. You could call Wellcraft in Sarasota and ask for Donny Waddoch (I may have mispelled his last name) He's a great guy and has been with them since Noah built the big one.
Good luck, have fun.