If one goes the header tank route
should one consider reversing the seal to keep the oil
in rather than the water
Years ago I considered getting rid of the cutlass bearing and prop shaft gland and replacing them with opposed tapered roller bearings in an oil
filled stern tube with a header tank to keep the seals pressure activated. It was doable and I found a couple of boat owners who had done it successfully however I gave up the idea when they warned me that it did cause noise
from transmitted vibration from the prop. Had I implemented it I would have set the seals to keep the oil in rather than water out.