As you just mentioned, if you have no intenet on saving the screws, welding a nut may be the best, the heat will certainly help.
If you are trying to save the screws, the "smack them with a hammer" style will be alot better than power driven. Since the hammer driven only does one shot at a time, the chances are slim that you'd strip the head. With a power driven style, if it slips onec, the head will be pooched.
I'd also try a cordless impact driver before I'd try air or electric. Much more control and not as strong either.
Any industrial tool supply should have the bits you are looking for. Ottawa Fastener or similar.