Ed's right. Sanding the comb front won't reduce the slide clearance on a Super 64. There is a back plate on these, but it can be left off so that the slide rides on the comb. In which case, sanding the comb makes sense, only to provide a good flat surface. When I removed the back plate, the slide gummed up more, so I reinstalled the backplate. I thought I'd tape the slide to reduce clearance, and discovered I'd already done that! It's still more leaky than my SCX 64, which doesn't have a back plate.
Reducing the clearance by modifying the mouthplate isn't just a matter of filing the bottom edge of the piece. There is a ridge, or channel, inboard of the bottom edge, that rests on the back plate, which is not easy to access. The least risky operation would be to tape the slide, or as Ed does, thicken it with paint. Tape won't last as long, but it is renewable, and reversible.
Tom