I'm about two weeks from installing a pair of marinized Duramax's in my project boat.
I want the prop center lines to be 3" below the bottom of the transom but the depth of the oil pan looks like it's going to cause me to raise the x mark up and ending up with a prop shaft almost level with the bottom more like a normal surface drive set up. Haven't cut the transom holes yet.
Are drive spacers a simple bolt on or do they require other additional changes?

Spacer plate is a relatively simple bolt on; need correct length studs, nuts, washers, o-ring, oil transfer tube and grounding strap. They are available in 1/4" increments from 1/4" to 3".

I have a set of 3" spacers for # 6's for sale if you need some.

They make dry sump d-max pans. That would probably allow you to keep engines, and center of gravity, lower.