To keep the shape of the beavertail grip safety, the frame is the part that has to be modded. It's pretty easy to see what exact parts need to be taken off once you hold the new safety against the frame. I recommend using a hand file instead of a hacksaw or dremel to do this work, as you have to be very accurate to make this work and still look good (you want the safety to fit flush w/ the frame etc).
I've done exactly this before on a project, taking a metal kit fitted TM 1911 from this;
to this;
