widest tire on 6 inch steelie?

Are you sure you have a 6" wheel ?

Unless it is a aftermarket wheel, there is no such thing as a factory Chryler 14" x 6" sbp wheel.
There was a 5.5" x 14" Rallye and Steel wheel.
And a 14" x 4.5" steel wheel.

If it is a 5.5" wheel, I have gone with a 235/60-14. Thats about as big as I would go.

On my 70 Dart I have 215/70-14's all around,on 5.5" steelies.
Very close to a F70-14 which was an optional tire for the 340 Dart/Dusters.