you might want to soak the plug with PB Blaster ( better than WD 40 but it stinks BIG ) for a day or so and start out and using an impact gun to try and break it loose but wire brush it good before soaking it as it will get in maybe -

when you install it make sure the threads in the block have been wire brushed real well and than cleaned with laquer thinner than you should use a thread sealer to install

Permatex High temp thread sealant - px # 59235
OR
Loctite 567 PST pipe sealant with PTFE High temp
for stainless steel and other Metal Fittings

i believe they are the same thing both companys its a paste
DO NOT use TAPE it will leak its for taper threads on plumbing pipes

hope that helps - jz