For a normal 5/6-point, you can use the stock outer-lower seat belt mounting holes for the outer lap belt mount, but you need to weld in reinforced threaded mounting plates for the other 2-3 (inner lap belt + anti-sub strap(s) below the seat). Some mfgs make alternate belt setups that make some compromises in the name of being safer and more stock-compatible without all the welding, e.g. Schroth.
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