removal of wiper wheel box nut

I need to remove the nuts from the wiper wheel boxes on my GT6 so that I can 
replace the gaskets.  Both of them are stuck fast.  I am afraid to crank too 
hard on them for fear of breaking something.  Does anybody have any good tricks 
for removing these?  I was thinking of trying an impact wrench on a low 
setting.  I tried penetrating oil but it didn't help so far.
thanks,
Dave