Up sh*t creek...

Soak it DOWN with PBlaster and let sit and repeat. Also tap on it around the perimeter with a hammer to loosen it up and help the PBlaster soak in. Keep doin that and it will come loose. Then after you unclog it, clean the threads REAL GOOD on both pieces and coat the threads with a good thick coat of automotive RTV like Permatex. Use RTV, not pipe dope. The RTV will both seal it and keep it from rusting closed again. Pipe dope dries out over time.