Don’t understand why the cylinders are wearing this way.

The challenges of boat ownership

Pictures below

I'd start by pulling injectors and have them tested and cleaned,is the oil clean?

Burnt Mark between cylinders

What did they set the piston to wall clearance to ? looks tight, what were the oil and water temps while running, might be a cooling issue with wrong head gaskets ?

The head guy thinks it’s from the timing being advanced to far.

Sorry I don’t really know that. I didn’t build the short block. I can ask and let you know.

Base timing on that engine is 8* then the computer does the rest

I did set at 8 on the base timing and even made sure the total timing was right as well.