Yes, width is fine...I just like te look of a taller tire....I love the BFG's and have had them on my 96 Sport and Jeep Wrangler but I have a feeling they would be a little rough on the ST....that's what I am shooting for...that look...
Either a 70 or 75 would work. I have 265/65/18's Pirelli Scorpions on mine and love them. 235/65's are around 30.05" tall. Mine are 31.5". 235/70's would be about 31" and 235/75's would be roughly 31.8". If you go to the 75's, your speedo will go from being fast to slow. Anything from 31-31.5" will true it up pretty close in most speeds. Mine is true up to around 90mph.