:thumbs: x2 on the "fire wrench" & candle idea, wish I tried it years ago. I had the same angled muffler fitting corroded in a H50 block and just sprayed it for 10-12 months every time I went by it with d40/liquid wrench etc... and kept tapping it with a drift and no luck.
So I figured I would try that old candle trick with another old school combo that an old farmer who never bought wd-40 told me years ago: before heating it and hitting it with a candle you mix up Automatic Trans Fluid mixed with acetone and soak it anyway you can for a while (I did a week with the whole engine tipped sideways so the head was submerged in the ATF/acetone mix). Once I heated the area around the fitting for a few minutes I put a cheater pipe on the pipe wrench and figured if I break the block, I brake it, I couldn't use the engine with the fitting pointing straight up anyway, and the fitting turned out of the exhaust port. (the engine was bolted to a frame for more leverage).
Disclaimer: Your results may vary. Do ALL of these steps outside away from everything the entire time, mixing these things together may have been done successfully for years but you don't want to take a chance, especially when you heat up that treated block outside. You might even not have to heat it up at all. I think mine would've come apart without the torch. I'm not a chemist but I stayed at a Holiday Inn Express last week...