Larry, trucks are advertised and sold with their wheelbase being measured from the front axle to half way in between the two rear axles. That's why you're seeing a difference. It is one that I didn't know until I joined this site.

In SD, provided you have sufficient wheelbase on the tractor, with a 42' trailer, you are legal for 12k, 34k, 34k which adds up to 80k. I punched your measurements into the table, and it looks like you should be legal for that. It's hard to be exact without taking all of the actual axle to axle measurements, but I used trailer length and subtracted to enter it into the table. Happened to have it open for my own use.

www.sdtruckinfo.com