The issue there is the slots are defined in two DIFFERENT places. Where you changed them, and in SpaceObjects_Xfiles.txt.
The later sets the maximum size of the arc. You can make it smaller via PurchaseShips and StartingShips, but not bigger..
And no, it isn't editing the ship file; just the text file. Look for lines like these:
Firing Point 1 Arc Start := 315
Firing Point 1 Arc End := 45
Aha! I had the same problem and now I could fix it... if my computer with Starfury on it were usable.
Oh well, when I get a newer and much more powerful computer I'll finally be able to make a Terran battleship with 4 forward-firing heavy weapon mounts! Muhahahaha!
It works! I've expanded the forward weapon arcs on a Terran battle ship to also face their respective sides and allow three weapons to bear to each side instead of just two. But it turns out that the slots are 'reversed' from what I expected, so I had to re-arrange the numbers to make the expanded arcs face outward instead of inward towards the ship...