68 Valiant dies and runs lumpy, then suddenly runs fine.
Generally drivability issues from cold start until fully warmed up are caused by incorrectly operating choke. Check vacuum pull-off for leaks if equipped, adjust amount of choke pull-off once engine lights off to about ΒΌ inch, check that bimetal spring is able to freely move choke plate, and adjust its closing tension with as little force necessary to just fully close choke plate. If closing force is too great full choke opening will be delayed producing an over rich condition for too long causing the chugging and fouled plugs you have been experiencing.