Being a veteran at removing twisted off bolts, I would suggest carefully center punching the center of the bolt and then drilling with a drill bit smaller than the full size tap drill. In this case, being a 1/4 x20 thread, I would first use a 1/8" drill bit and sneak up, depending on how well you centered it to a #7 drill for retapping the hole. What I found really works well is to use a left hand drill bit, which if you're lucky will unscrew what is left of the bolt and you drill it out. If your drill hole is not completely centered, you can use a burr and a die grinder to re-center the hole. Usually on these smaller sized bolts that are rusted in, as yours appear to be, an easy out is not going to get the job done. Seeing how rusty your truck is and how difficult your screws were coming out, I would've used a left handed drill bit and used the phillips head of the bolt as a center and just drilled the bolt out and run a tap through the hole. I hope when you fix everything that you use some anti-seize on fasteners where appropriate.