Radiator drain & gas tank leak question

The drain petcock should be an NPT (tapered pipe thread) size. If you just want to put a pipe plug in, go to the hardware store and grab a couple different sizes and see what you can find. Make sure you use some teflon tape. Pipe threads are different thread pitches than standard bolts, which is likely why you couldn't fit whatever you were trying to thread in there. As far as the gas leak, please, please, please don't drive your car until you fix it. My old '65 fastback had a split hose that connects the filler to the tank and it leaked. I think they're less than $10 from NPD and it could save your life. Even if it turn out to be the tank as the source of your leak, they are cheaper than a day in the burn center or the current price of caskets. Gas leaks are never a place for "good enough".

The difference with NPT threads is that they are tapered; that's why a regular bolt won't fit. The thread size on the radiator drain is very likely a 1/8-27 NPT. That's all I have ever seen in the all-metal radiators.

And yes, replace the filer neck hose on your gas tank. Now!

Once you get the tank siphoned out(or drained it has a drain), it's not even a one-hour job to replace it. When you're ready to start inspecting the tank and filler neck, go ahead and get a new filler hose and cork gasket to go between the filler neck flange and body. You'll need to replace both whether the tank is bad or not.