I am sure you can get wider than an 8.5" wheel. Maybe even a 10". I don't have my notes anymore, but I have 16x8 wheels (4.5" backspace - neutral offset) with 245/45/16 tires on my stock trailing arm (relocated e-brake cable). If teh offset arm gives you another 2", then you should be able to use a 10" wheel, but the tire will make a difference as well.
Van Steel says " you can use with the popular 10" wheels. That means that arms can accept 2" over the standard tire size and still give you the same recommended factory alignment. " here
http://www.vansteel.com/Rebuild/offset.html
Remember, you will need a shorter spring if you go with the offset arm and fat tires. mine come pretty close to the spring as is.