If this were my project, I'd start by measuring the width of the opening on the top of the track. That will determine the max drill diameter/countersink diameter you can use without drilling into the upper lips of the track.
If using conventional 80º-82º flat head screws, the countersink diameters will be approximately:
.656" for a 5/16 screw
.531" for a 1/4 screw
.411" for a #10 screw
.359" for a #8 screw.
I'd then find a drill bit the width of the track opening and use that to lightly mark the center of each hole, I'd do this on a drill press. The drill bit chosen will ideally fit just between the 2 upper lips of the track and will then be centered in the track opening. Just a light amount of pressure is necessary as you are only marking the hole centerlines and creating a slight pilot.
I'd then drill clearance holes for the size screw you've chosen to use, using the hole centerlines as a guide/pilot.
Finally I'd add the countersinks for the screws. The countersink may be a center drill, a spotting drill or as mentioned earlier, a drill bit reground to incorporate the correct angle. Drill bits are usually ground at either 118º or 135º and you will need to be at 82º.
The nice thing about a spotting drill if you can find a diameter that will work, is that they are available in 82º and are also available in 1/4", 5/16", 3/8", 1/2" and 5/8" diameters.
