I assumed and that is always dangerous, that piston pin location is to remain standard. There are so many different pistons available these days mainly for racing apps that nothing should be assumed. If using a stock piston the top of the piston at the perimeter edge will be the same
distance up from the pin whether it is a flat top or dished. Paul is right about that. One item I would disagree with is the effect of an overbore on compression ratio. It will raise the CR a little because you are squeezing a larger volume of air into the same size combustion chamber space, so the ratio goes up. The amount is almost negligible sp? but when added with other small changes it can start becoming a factor. Using a .040 head gasket from FELPRO is a very smart choice and will give excellent sealing but since it is thicker it would raise the head off the block deck that much. That would increase the head chamber volume a little and would lower CR. Bottom line: The .030 overbore will raise the CR a little and an .040 gasket would lower the CR a little therefore the final CR would be relatively unchanged. DO WHAT?
Best advice so far.
1. Consult piston manufacturer, use the best piston you can afford
2. Stay modest with cam selection, not too much duration over stock or you will lower cylinder pressure at lower RPM's
More lift is OK
3. Find out what your head casting numbers are to determine chamber volume
4. Brass freeze/core plugs
5. Concentrate on making TORQUE, not horsepower. Torque is king on a boat engine. A boat engine is always pushing uphill
6. HAVE FUN