Unless it's important to you don't worry about the seat belt light. I probably shouldn’t have said any thing about it.
The best harness for you is a 71 SS or Monte warning light harness. A 70 is almost the same, there are a couple of differences but it will work just fine. You can use a 72 dash harness but because of the numerous differences between the 71 and 72 forward lamp and engine harnesses, you will need to move wires in the bulkhead connector to make it work.
Where the wires pass through the firewall. The forward lamp and the engine harness plug in the fuse block (dash harness). This is a bulkhead connector.