There are 4 main areas where slop can be:
1) the bushings on the transmission
-This can be fixed by doing the age-old, tried-and-true skateboard bearing swap.
2) the cables themselves
-Unless they are really worn out, the amount of slop in the cables will be minimal and the only way to fix this is to get new ones.
3) the bushings that isolate the shifter assembly from the chassis
-These can be replaced with solid counterparts (symborski's)
4) the plastic bushings that are in the shifter assembly themselves
-The plastic bushings mentioned above are most likely your primary source of slop. I found that 3kgt/stealths have a wave washer in their shifter that does wonders for tightening things up even with old, worn bushings. When I made my short shifters, I also custom made new bushings. I got my final fitment turning them on a lathe with sandpaper, but that might not be an option for you if you don't have the right equipment. Of course, you can always buy new stock ones, but they do have some tolerance built in.