You've got to do what you feel comfortable with, but I don't think money should get in the way of safety. I started with 100# Irwin clamps and quickly went to the 600# vises. I feel a whole lot safer using the larger clamp, especially when pouring multiple molds. It seems to me that the molds would have to be centered in the bench vise bench vise. If the spru is on the end of the mold, you would be applying downward pressure to an unsupported part of the mold. Perhaps unlikely, but that would be an accident waiting to happen.