Best tire sizes

Joe, you can definitely go bigger...the limitation will be on those wheels first. 185 is probably about right for a 6" wide wheel, you could go a bit wider on a 7" wide wheel...

Before you do anything, it sounds like you know how much space is there with your current setup. Armed with this data, this tool should tell you everything you want to know.

Wheel-Tire size comparer - RIMSnTIRES.com