Found the answer.

As far as the pan, it will come off by itself. No need to mess with timing cover. As for the gasket it depends on which gasket was used if it will be reusable or not.
Setting full mark on the stick is another thing. Every GM engine I've ever owned showed well over the full mark with proper amount of oil in it. Best thing to do is put the proper amount in, put the boat in the water so it's at it's normal attitude and run the engine a few minutes and shut it off. Let sit a couple minutes. Pull the stick and see where it's at. Then mark the stick with a scratch or center punch.

Thanks Smitty. It’s a blue Fel pro gasket. I have several spares so really what I was wondering was if the pan could be removed without much more effort than pulling the bolts and if the gasket would be destroyed in the process. I will use your advice for setting up the full mark on the dipstick with boat in the water. It’s a 9 qt. Offshore pan. I will be removing the oil cooler, oil Tstat, and associated lines in order to install a remote filter setup at front of motor. So my guess is I will need to add about 10 qts. in total after filling the new oil filter. Agree?

I'd start with 9 and see where it's at. It's quite common to run one less than what they total system can hold to help with windage control.

Does the pan have a built in tray ?

Yes it does.

Since pan is coming off , check level by filling to just under tray. This is how peeps did it many years ago, check around to see if that practice remains the same.

You can mark the stick once you know correct level.

For some reason, seems manufacturers are off sometimes on there pans capacities.

Double post.

Great idea, thanks SB.