Bell Tech or Western Chassis on the spindles. Not sure what brand Switch Suspension uses but they are one of the newer know sources.
Find someone who does an under bed notch that has more beef to the brackets vs. a Belltech or Western Chassis standard bolt-in c-section.... especially if you keep the leafs & tow. That's a lot of stress on that section of frame for it to only be 2" tall. I did a step notch to keep the rail @ full height but it requires raising the bed floor (or cutting it out for a sheet-metal cover).
Lowboy Motorsports is a well known source for 'big-truck' wheel conversions on Squarebody/OBS duallys….
http://lowboymotorsports.com/
Wheel sourcing, cutting, machining, adapters, & mount/balancing for tires. He ships all over the country if needed. Also has a Squarebody dually kit for optimizing placement of the wheels in the fenders (arms, steering pieces, shock relocation, & joints).
Nice looking purchase!