The issue with Highway 40 Blues is that the tag’s quick chord changes don’t land on the standard quarter-note beats. I would notate the rhythm as two dotted quarter notes, another quarter note, and then a whole note. Or if you were to count it really fast, it’d be “123 123 12 1” or “A23 C23 G2 D” (spaces added for visual clarity).
Strum Machine will hopefully support this kind of syncopated strum in the future, but it’ll be awhile!