i have an imco sc upper and lower and wanting to upgrade to scx upper and keep sc lower. does the scx move the drive/prop back 3/4 inch? aka prop deeper in the water? wondering if i will need a 1in shorter lower with a 1/2 inch spacer? asking because i want to send my standard sc lower to wilson to be shaved and blueprinted.

I don't know if it moves it 3/4" back, but I do know that it won't make a difference. Normally with a 12" ext box, you go to a 1-2inch shorty.
If you set up is ideal now, then don't worry about going up.

very good point