73 Duster - Need Help

I have 225/60/15 up front and they work great never had any clearance issues but they fill the well nicely. In the back I have 15X7 rims with 225/60/15s on them, but I really want to change that because there is a a lot of space around them. I have HD springs on there and I would imagine a 255 would easily fit. I don't know about the 275 though, maybe with S/S springs you'd clear it. I am not a big fan of air shocks because they stiffen the ride way too much and you can get the same clearance but better performance out of S/S leaf springs.