Leveling the boat is a calculation called 'moment weight.' (weight X arm)
If the engine is 400 pounds 100 inches aft of the mast
and the outboard is 200 pounds 200 inches aft of the mast
the moment is the same (40,000 inch pounds) although the boat is 200 pounds lighter and the boat should stay level.
To add additional 200 pounds to regain the original total ballast you can add 200# at the mast. if you added 200# of lead on the engine bed
you would have the original weight but the moment would be 20,000 inch pounds aft.
Depending on the numbers you may want 100# of chain in the locker and 100# on the engine bed
. You could add batteries - you could do a lot of things.
Measurements forward of the mast would be "negative moments" but there is no secret to establishing a datum, you could measure from the bow and everything would be positive.
I don't think you can do any real harm here, I am just tossing in some thought.
Racers would prefer all the distributed weight be located as low as possible in the center of the boat as a general rule
It should also be easy to create a frame or platform using the original engine bed boltholes for a secure lead installation
. If you have a "significant" amount of weight you really need to make sure the structure you attach it to can take the loads. You know the engine bed can take 300-400# pounding along day after day.