Its extremely hard to find nerf bars for the square body because the gas tanks are outside the frame rails, which gets in the way. All the newer trucks use relatively short, flat brackets to attach to the frame, which will not work with your truck.
I have considered trying to make a longer bracket that would let me adapt the newer nerf bars, but its one of those projects I never get to. The other option might be to get an extremely long (front to back) nerf bar where the brackets would be in front of and behind the gas tanks, but that's a long span and would require extremely stiff steel with no center support.
I have small hoop steps mounted under my K25 cab and they work fairly well. They mount on the lower edge of the cab body, and you can barely get to the bolts because the gas tank is so close to the body.
Bruce