85 vs 75 tires on dually

I have a 1997 f350 dually and it calls for 215s. I bought it used and it had 235s. So when it needed new tires I went with the 235s and never had any problems until I tried some Goodrich commercial t/a tires. I had always used Cooper tires before but after seeing so many good reviews on the Goodrich I tried them. They were so soft that I had to run them at 80lbs to barely keep the duals from touching when loaded. Eventually I had to put 1/4" spacers to feel safer. With the Coopers I can run at any pressure and have plenty of space between them. Both tires were 235-85x16s. The Goodrich may be good on a four wheeler but not on a dually.