I am changing the overflow valve and the one of the bolts and nut is stuck,
I have tried wd40, and heated it to see if it would loosen, its holding up the process.
What can I do to get this damn bolt and nut alose?

Hi, a friend ended up using a hacksaw on my rusted toilet bolts and sawed them off. You have to get a saw that cuts metal and it'll take about 20 minutes. Maybe. Hope someone has a better answer for you!

Take a hacksaw blade and wrap a rag around it for a handle and saw that sucker off between the tank and the bowl. That's what we have to do with a tank bolt that won't come free.
Good luck, Tom