truck and trailer question

i had a 379 and a 50' step and i was a few inches short of 73'

 

The 379 has a bumper to back of cab length of 127". Add the 63" sleeper. Estimate or measure the steer axle set back. Without explaining my figuring, I am going to guess about 60 inches from back of cab to trailer with coupler set about 6 inches forward of center, and an 18" king pin.

The minimum clearance between back of cab and corner of trailer in jackknife position is four inches. This four inches added to the swing radius in the table gives the point closest to back of cab at which a fifth wheel may be mounted. This table is four 96" trailers. With an 18" kingpin setting and a trailer with square corners, the swing radius is 52". Adding the four inches of clearance plus three inches for a wider trailer, you now require 59" from the coupler to the back of cab. You have 68" with the fifth wheel centered. This is a wild estimation.

 

475 cat 18spd 3.55 rears 5.5-6 mpg

 

Off the national network in your state, I believe combination length is restricted to 65'. In my state, trailers are restricted to 50' off the national network, but combination length is not restricted.