If you can get away without using the Hasselblad bellows lens shade you might consider a B50 to screw mount adapter and a threaded ND filter where there are way more options to choose from. (the beauty of the B50 is that they will still attach to the lens when the lens hood is attached, with a step-ring adapter you lose that option)
Having checked an old Hasselblad catalog I see they produced 4x, 8x and 64x (2 stops, 3 stops & 6 stops) ND filters in the B50 size.