real nice seat job, Bob - no way to change anything that'll give ya more room, especially after all that work on the seat!! but if you find a wheel from a 55/6 TF truck it'll give you a couple more inches - the 55/6 trucks had a slightly longer column, so the wheel was flat, rather than the the 'dished' style we got 57-9 - of course you could always cut a couple more inches off the column, or off the driver!
Bill