Before you go tightening that bolt too much, have a read here:
I stretched that 1st bolt almost 1mm when compared to another 2 factory bolts. I then purchased an ARP bolt from Spool Imports but it did not come with torque specs, so used the recommended for a M16 bolt on the ARP website. Car is running now no issues yet. I just checked it last week.
With regards to the MIG welding, I'm no expert welder but a MIG does not penetrate very well. It would be better if you can get your hands on a TIG.