How many meat birds? Will you be brooding them in the tractor? How will you be moving the tractor? How often would you be moving the tractor? Do you expect the use of the tractor to provide a large percentage of their feed needs, or will you simply be using it as alternate housing to a stationary coop? Do you have option to run an electric fence around your pasture to keep predators out? Do horses use the pasture? (in which case, they may help with some predator control???)
You could go with cattle panel walk in style. or build a shorter tractor that is 2 - 3' tall. My most recent build was done in panels, connected with nut/bolt/ L brackets, and used lawnmower wheels. It gets the job done, but I'm not overly pleased with it. I still use it every year, especially when growing out chicks.