sir
my experience in 20 years of water industry would lead me to try
first soaking overnight with every breakfree style stuff you can find
we use "certified "brand break free spray
if all else fails
mark the center of the bolt with a hole punch
using a cobalt or diamond bit drill the center of the bolt out the
depth of the bolt...try more break free ...if that failsl you can drive
an "easy-out " bolt remover into it and try that....if that
fail....oboy......drill the bolt to the diameter of its threads and
retap the hole
sound like fun...hahahahahaa
stainless is famous for spalling (welding) itself so always use
Neversieze on stainless fittings

vitus