Every one I have ever played with was 3/8-16. Never seen a steering wheel with 5/16 thread and definitely never seen one that was metric, and never had to helicoil those holes though I am a bit more familiar with helicoils than I should probably be.
Interesting. My 84 Prospector is tilt and uses 2 holes for pulling the steering wheel. After putting 5/16-18 helicoil threads in I was able to pull the wheel. It took about 12 'wop's with a 4 lb bfh to pop the wheel loose. Without the hammer effect those threads get pulled out sometimes.
I have never had to helicoil one yet. Apparently, someone used the wrong bolts at some time. Never had to beat one off with a bfh either. Get a scrap of metal and drill the right holes in it if need be. I have done it before. Better than beating the crap out of it and spending time and money later trying to fix it.