Wilson advertises kp 22" from front of trailer and rear axle 24 13/16" from rear of trailer on a 41' model which puts rear axle to kp at 445"  If your 5th wheel is 16-3/16" forwad of tandem center you have 624" overall bridge or 52'0" axle 1 to axle 5.  You could easily drop back to a 183" wb tractor and still bridge 80,000 with kp 16" forward of tandem center.