Was hoping to get around 80,000
I don't see how you could get to 80,000 if you have to comply with federal bridge law. Unless you use one of those rear fold down axles like the concrete redi mix trucks have, to stretch it way out and also add a bunch of axles.
And just so I understand what you want to do- are you wanting to cut and stretch wheelbase in the middle for the bigger box? Or full rerail to longer wheelbase? With the setback front axle and the current wheelbase you're pretty short to get any bridge.
How long is the box you currently have? And how tall could you make the box and still get it loaded with your equipment? Are you only loading at a quarry, or are you wanting to load it with a mini x or skid loader?
There's some trucks around here that run short box with some real tall sideboards, quarry to delivery. Loader at the quarry gets over the sideboards fine.
With the current wheelbase, you're probably only legal for 50-52,000 gross for federal. (just guessing at wheelbase)