Would a 75 Duster front end kit fit my "updated" 68 Valiant?

Yes in a nutshell. If kit is only upper and lower control arm bushings and balljoints. Tie rod ends are also the same. Possible differences in pitman and idler arms which should be ordered to match the year of the "K" member you are using. And yes you will need BBP disk spindles, upper control arms, and a disk/drum proportioning valve as well.