My 96 T100 has the Extended cab. I need to move a nice Steinway Upright 700 miles, but the geometry of bed/axle concerns me.
Will the weight of the piano behind the axle cantilever the front suspension to the point of poor control? Wished I hadn't sold the F250 last month.

 

I don't believe at 600lbs . You should be fine.

 

You'll be fine. To get some perspective, think about whether you'd be worried about overloading the truck with 3 guys sitting in the bed at 200 pounds each. You probably wouldn't think twice about the weight (maybe about the safety). Strap it down so it doesn't move and the weight won't cause an issue.