3-Point seatbelts into a '64 Dart hardtop?

Thank you all for your responses. I may still end up welding the reinforcement plate in, not sure yet. If I do I will try the technique chinze57 mentioned of welding small portions at a time and letting it cool.

Kesteb - Thank you for the link to your installation. I'm not sure if I will be able to implement any of your documentation though as my car does not have a B-pillar.