66 Signet, 14" Rallye wheels- how wide a tire can I fit?

14 or 15 inch? How wide? I have 255/60 15 tires mounted on stock Mopar 15x6 on the back of my 66 Valiant, 8 3/4 rear and Mopar S/S springs and they fit great! On a 14 inch rim you could go with a 235/60 14 or 245/60 14 and shouldn't have issues.