Where are you pouring the pewter.....front or rear?
At the rear, it shouldn't be a problem.
If it's at the front, I always drill a hole instead of driving the spike in. Then cross-drill a couple of holes thru the antler and tang. Then incorporate some type of design which extends past the holes on both sides.
When you do your pour, it fills up the main cavity, the cross holes, and whatever design panels you put on the sides. The pewter acts as hidden rivets to hold the blade in.