Anyone know the correct x-dimension for #3A Speedmasters on a 38' Scarab with straight bottom

The X-dimension on my 38 has the prop shafts even with the bottom. I have a notch transom on mine and am running #5's with a 1 inch spacer to bring them down. Any higher and handling is worse without any significant speed increase. Any lower and the top end goes down. If you don't have a notch you will need to run the drives lower than mine or use an extension box. Try Competition Marine Center in St Jo (636-946-9140). I think they have some experience with 38 Scarabs and they are closer to you.

Jody165 and I both have stagger setups which will affect X dimension, as well as transom, etc. I have #3's with 18" of shaft separation. The propshafts are roughly even with the bottom of the V, or about 4" below the hull ahead of the drive.