How Much is Your Life Worth?
Please oh please take a look at what you asked!
Follow the instructions given by the seatbelt manufacturer TO THE LETTER!
Welding is not required in many cases, just that you have sufficient doubling of the attachment point to prevent tear-through. In many cases, the Stock seatbelt anchoring points are NOT acceptable for mounting of the lap-belt portion of a multi-point restraint system! They all come with instructions, and offer technical support. Diest, Simpson, Schroth Those systems are in my cars. They are all outstanding.
When thinking about seatbelt anchoring, keep just two words in the back of your mind: "DALE EARNHARDT"