I discovered that it's better to put all the smoothing filters before the subtitles so that they aren't blurry
Code:
#=====================================================#
# -= AviSynth script by MovieStacker v2.0.0 (beta) =- #
#=====================================================#
LoadPlugin("C:\Program Files\video tools\MovieStacker\Filters\MPEG2Dec.dll")
LoadPlugin("C:\Program Files\video tools\MovieStacker\Filters\DustV5.dll")
LoadPlugin("C:\encoding\textsub.vdf")
LoadPlugin("C:\Program Files\video tools\MovieStacker\Filters\LegalClip.dll")
AviSource("D:\encoding\MOVIES\movie.avi")
LegalClip()
SpaceDust()
TextSub("D:\encoding\MOVIES\movie.srt")
BicubicResize(512, 334, 1/3, 1/3, 1, 0, 638, 352)
AddBorders(8, 73, 8, 73)
LegalClip()