I went and removed the one from my 1987 Chevrolet Caprice and measured it. Hopefully GM has kept them similar and if they have it's a 8mm x 1.25 thread. The length is about 2". This is a bolt, not a nut. There is also a flat washer on it, no lock washer. If the hardware store offers the bolt and washer in stainless steel I would go for that as it won't rust up on you.
You will need a 13mm socket or wrench to install it.
I hope this helps.
