The original question has been asked before but as many threads on here it went off on a tangent about 440s...
Did any 426 wedges show up in early 66 cars?
The dumb question is the one you should have asked.
They look like this. Found under the carpet, behind the back seat, taped over the glove box, in the head liner. You never know. Sometimes they're not found in the car at all.
I can barely read the "426 Wedge" at the bottom of the sheet. Maybe take a couple closeups of the engine code area and a picture of the fender tag would be great.
Seat belts weren't mandatory until 1968. Lots of cars didn't have any. Here's a good company to buy from.
Replacement Seat Belts | Lap Belts | 3 Point Seat Belts