I have a 2003 Merc 350 MAG MPI 0M300000 Bravo that I am prepping to do head gaskets on. My question is, what automotive engine does this engine cross reference with? (yes I know not use steel head gaskets)

Thanks in advance for any help given.

A chevy 350. What do the valve covers look like, hold down around the parameter or 4 in the middle ( vortex) ?. Vortex were worth 55 hp if I remember correctly but the intake manifolds are different.

It has 4 in the middle. It is the 300hp 5.7L vortex. It also has fuel rails on each side of the throttle body and a split intake manifold.
I mainly want to make sure all my ducks are in a row and I can get parts in a pinch at a local autoparts store if need be, before I start tearing into it.

Also depending on year the vortex intakes are different.

4 center bolt valve covers doesn't automatically mean Vortec.

Look for 8 vertical intake manifold bolts. 4 at each end and none in the center. If you don't have that. You dont have a Vortec motor.

I stand corrected, I made an assumption that the other versions of the 350 were not installed stock by mercruiser.

No biggie. Mercruiser made the leap to all Vortec way after GM did.

Two piece intakes with plastic top, aluminum bottom are Vortec.