boy, if you turned it out with that short a leverage then it had to be the welding heat that loosened it up. maybe we should touch the arc welder tip to every head bolt before we attempt to remove. Where did he put the ground clamp?
Great idea and great success... however just some thoughts.
If the ground clamp is on the block and using a -ve earth, the current passes through the block across the thread, up the bolt to the electrode. The corrosion may be resistive and will generate a heat spot too. (+ve earths the current flow is the other direction)
The heat may help break the corrosion. If there is to much heat in the thread area, it may further weld it on. (more likely on cast).
Also, becareful if the current is passing though bearings. It can "mark/pit" the bearing surfaces and they will need to be replaced in time.
This is not a critism of what was done, just some shared learnings.