I've always followed, nothing bigger than a 255 for dual tire sizing before a spacer is needed.
Mink is correct though, you want an adapter style spacer that has holes for your current studs and studs on itself. I've been running a pair of 2" hubcentric steel spacers between my rear duals for about a year now without any ill effects. I've towed heavy and haven't done anything differently than I did before the spacers, without any problems.