An air chisel with a sharp flat edge bit on the shank near the head of the bolt will walk those bolts right out in a few hits. No need to remove the steering shaft. If you have no air chisel a long cold chisel, and hammer will do the same thing. The chisel needs to be sharp to get a bite on the bolt shank.
Pull the new ones in with a non locking nut with spacers as has already been mentioned. Lube the threads as well as the serrations to aid pulling the new bolts in. Most all GM bolts are a 7/16" coarse thread.
Old flat rate mechanic trick, time is money when you work flat rate.
If the frame brackets will not hold the serrations to keep the new bolts from turning, a couple spot welds with the bolt in tension will suffice.
Just a suggestion but that's how I'd attack the problem.
