2350 Onan Generator spark arrestor cleanouts question

My 2004 Ford E-350 based 2350 has an Onan KY generator.  The manual for it says to remove the two plugs in the muffler every 50 hours and run the generator to clean out the spark arrestor.  My question is, how does one get those plugs out?  On mine they are 1/2" square heads and are stuck in place.  I can only get to one of them due to the heat shield. I've tried penetrating oil and an 8-point socket with a breaker bar and it won't budge.  I'm afraid of stripping the heads or breaking them off.

Has anyone done this?  If you have, how did you do it?  Thanks for your input...

I’ve replaced the Carburetor, replaced plugs, filters and the other day the starter. Mine has 347 hours on it and I’ve never done what your referring to.

Myself I wouldn’t bother with it but if I were to take it on.   To remove the entire exhaust only requires disconnecting the hanger strap and loosening the u bolt clamp directly under the gen set itself.  All of a 3-4 min job. From there you could set it on a work bench and use something like kroil oil to loosen the plugs in question. 

Warning if you have never used Kroil oil take heed.  Do NOT use this without gloves. It’s called the oil that creeps for a reason. You do not want it in your skin.

That’s the approach I would take if I were so inclined to attempt this maintenance procedure.

Thank you!