If you add halyard turning blocks to the mast, be sure the fleet angle is as close to zero as you can reasonably get. If you have to mount it/them on the curved part of the mast, use the split footed type like that in Chris' (holdech) pic. You can gently bend the feet to conform to the curve in the mast. Or use a hinged unit, but they're much more money for the same series block, and not necessary IMO, as they're really intended to handle changes in line direction, whereas a halyard always follows the same line.
I would avoid mounting it to the deck if at all possible, as any penetration is a potential leak point, and a way for water to get to the core. I would also try to keep the plate, and notch it for the hinge pin, as you never know what you may want to do in the future. If it's not in the way, then there's no harm in keeping it.