Does the gasket appear bulged or ripped? You might be able to snug the nuts and stop the leak, but only if you're lucky.
Do not start this job at 8 pm when the hardware store is closed. Good idea on the Kroil, antiseize, and new nuts...but there is always a risk one or more studs will break off. Are you prepared to drill out the old studs and tap the boiler in the event a stud breaks off?
Start spraying the nuts/studs with Kroil right now, 2-3x per day. Before you bust out the wrenches, I would heat the nut with a torch and give it another soak with Kroil as it cools.