Use an M931(A2). It's the short frame length, so no frame cuts needed. Study the rear suspension, use springs on the outside of the frame and put air bags for a load. You can vastly improve the ride and when you hall a load on a trailer, have the air already in place for trailer air brakes.
I sold one (a 931A2) that was a beauty and would make the perfect bob. He's wanting to sell it and offered a great deal to me.
I had an 817 that I loved.....would buy another...or trade. Absolutely don't do it to an 817/818. The creature comforts of the 931A2 are worth it.
98g is spot on about tires.....for play go big but for work, go 395.