JSC,
You could not use a 300 on your bike the way it is now. Your 280 is already rubbing the frame on both sides of the tire where the shocks mount. A 300 is both taller and wider and you would actually dig into the tire and bite chunks out of it. The only way to not rub your tire other than putting struts on and having 0 suspension or putting some air shocks on and keeping them aired up so hard that they feel like struts is to actually widen your frame. This will allow you to have some suspension travel since the tire would then be able to ride inside the frame where your shocks mount.