once you use a spot weld drill, such as a rotobroach or the ones sold at autobody supply houses, you can carefully use an air chisel to help "pop" up the hinge off the cowl panel. don't plan on reuseing it, as it will be killed from this process.
after you get the frame off, be sure to prep the cowl panel accordingly, filling any holes by welding/pacthing as needed. sand it smooth, maybe some rust neutralizer if you'd like. any holes in it, will cause water to get into the cab and cause more rust. then weld your new frame and hinge one.i found it i weld the hinge to the w/s frame then get it positioned correctly onto the cowl, it makes it alot easier.
word of advice:
as you are about to break out the hinge,have someone there to help support the frame, as it has a tendency to want to fall back into the truck atop the steering wheel.