Some of us swapped the bumper completely on our 987 with a GT3 aftermarket bumper. Mine is sold, I'm sorry I can't help. But you can either approach the sponsor here who sells them or wait for a current owner to chime in.
Leave enough space for the type of underside clamp you are going to use as a minimum. I think the actual GT3 gap has shrunk over the years. I set mine to 7/8 inch and it looks OK to me but I think that is about the max you'd want to go. That's 7/8 from the frunk opening to the flange of the vent, NOT from the edge of the hole you cut in the bumper.