Old Iron-
I looked a few places on the 'Net. It appears to me that welding is fine on 99% of trucks you and I would come in contact with.
It's only on certain very heavy trucks, such as the semis we see, that the frame is heat-treated. On those, they should not be welded.
I would be willing to bet that NONE of the 'bolts out there have heat-treated frames. Might be a good subject for discussion or tech tip....
