I also run a 18x10 rear on my '69 with 5.625" b/s and a 275/40 tire and my car eats that up whole,could have easily run a 285 or larger...but then again,my car's a freak.
I've put the 18x10 (5.75" b/s) & 305/35's off of my Chevelle and those fit with clearance...weird car I tell ya.