I just finished building my first on the northern tool 5 x 8. I built it to have a 5' wide interior and it is 10' long with all the extra length built out onto the tongue. A tongue extensions was necessary, but there is still room to mount a tongue box if I need one.
I was able to get a full 5' interior on the northern tool with 1 1/8" thick walls and without having to do anything special to deal with the fenders, (although they do appear to put a little inward pressure on the walls).


I left the axle in the standard place and with a battery, window a/c, PD4045, and 40qt 12v koolatron in the back we have a tongue weight of about 130lbs. Although, the axle wouldn't be that hard to move using a decent drill, titanium bits and a can of wd-40.
Also, you may want to check into license/registration in your locale. The Northern Tool I got came with a manufacturers certificate of origin, which at first made it more difficult to register (because nobody knew what to do with it) but ended up saving me time, money, paperwork and a "safety inspection".
The northern tool manual says max 45mph, the tires it comes with say max 55mph. It took 55 just fine, I don't know that I'll ever bother to go faster - for us it is all about the journey.