If you slide the fifth wheel all the way back how are you ever going to get any load on the steer axle?  Steer has to have 12k in order to make it to 80k gross. 

Inner bridge 2-4 is exempt from the formula as long as it is a minimum of 36'.  It would have to be about 39 if not for the exemption.  The exemption doens't help any unless you have a half way long tractor because you will run over the 1-5 bridge. 

Our short tractor has to be on a 42' trailer to be legal at 80k.  The longer tractors can pull the 38' or 40' and still work.