If you fill the cavity with pewter, I fear the knife will be very handle-heavy and awkward. Consider filling most of the marrow cavity with epoxy or wood.
As to the rest: Take a file and reduce the thickness of the bone by about 50% following the marrow cavity - I'd make this depressed area 1/4-1/2" in width . In doing so, make the front edge higher than the edge closest to the full-diameter bone (file a groove or several grooves). This creates a ridge or ridges that lock the pewter bolster in place. I'd also file a few V-grooves in the tang to lock the blade into the bolster as well.
It is also recommended you warm the area with a heat gun before pouring the molten pewter as it facilitates the flow of metal.
After pouring & cooling, dress off the pewter flush with the outer surface of the bone.