To give you an idea, here's a couple of pics of my truck in '09 when I had a 62" WMS to WMS 10-bolt under it. The rims are 8" offset towards the outside. Tires tucked nicely with adequate clearance on the inside.
I suggest just welding on the perches and getting it rolling. You can always add the ladder bars if you build the monster motor. Meanwhile, spend the money and effort elsewhere.