I've not seen too many pickling jobs that worked real well. You wind up with trapped air or water that leads to a rust spot on a cylinder wall, or bearing, or a cam or something.
Detroit's are easy to get running again:
Change the oil, engine and gear, don't worry about the filters yet.
Remove the air box inspection plates
Take the valve covers off and put pennies between the rocker arms and the valves
Loosen the exhaust manifolds
Re plumb the fuel supply and return to run out of a 5 gal. can
Remove the starters and go through them and reinstall: fresh water rinse, dry it out, then lots of CRC.
Disconnect the alternator
Turn it over by hand a few times to make sure the cylinders are clear
Motor the engine with the starter to blow things out for a little bit
Replace the airbox covers
Remove a couple of the pennies at a time and she should fire right up, take out the rest of the pennies
and the remaining cylinders should fire, it is going to smoke like heck...
Let it run a minute or two then shut it down and re tighten the exhaust
Fire it back up and let it run, keep an eye on the oil and don't let it get milky
Exercise the gear if the boat is in the water
Your going to have to change the oil a few more times, and the starters and alternators will likely need to be rebuilt/replaced but internally the engine should be fine.