Slasher won't fit heavies, it has very low fitting resources even for a frigate. Light missiles (standard launcher) are probably the best option, otherwise it's rockets (really short range, but higher dps). Light missiles can hit out to 42km with max skills (18km with no skills); training the skills janek suggested will do the trick. Rockets have an absolute max of about 10km.
Of course bear in mind the slasher only has 1 missile hardpoint anyway... If you want a fast missile frigate, you may be better looking at the caldari condor (2 missile hardpoints, but not as fast as the slasher) or the minmatar breacher (not that fast for a frigate, but more missiles again).
Also, go for a microwarpdrive if you want speed, ideally with an overdrive injector in the low slot (or a nanofiber internal structure - not as much speed increase, but faster warping and turning). With those, a slasher can get well over 4km/s!