I just removed my plugs (87 GN). They are 1/8" NPT threaded, brass plugs. I replaced them with conventional brass radiator draincocks.
My headers & front wheels are off, so it was easy to reach the NPT plugs. With everything in place, it's hard for me to visualize, but it could be a lot harder to access.
Even though the brass plugs have a square head, the soft head rounded off when I tried using a square ratchet socket -- the plugs were in tight. I ended up using a brute force, vise-grips to get them off. On new plugs or draincocks, even if they use "dry-seal" NPT threads , I wouldn't trust it -- I applied thread sealant.