Nice find. I would consider adding the stud kit.
The 3 side bolts shouldn't be too bad with a good soak of pb-blaster or the similar.
The bolts on the flange are going to be a "dog of the female gender".
If the flange bolt heads are intact, I would just try and soak them to see if they will loosen with a bit of heat. If that is no-joy, since you are replacing the manifold I might be able to just twist off the heads with a 6 sided socket and breaker bar... or if you have one an electric impact wench.
Then just get a block of wood and hammer and tap off the flange. Check the the condition of the flange, and if it looks sketchy... this is then a perfect time to replace it and the exhaust stack.
