Tex, I did the same job in 2016. Not fun, but not really all that hard either. As for your questions: Remove the single nut on each of the six studs that hold the manifold to the engine. Remove the entire manifold (yes, it will have to be completely drained first, with all the hoses removed). Once the manifold is off, you will have more exposed thread on the studs and be able to do the double-nut removal trick (may require soaking in PBblaster first).
Removing the six studs doesn't release any anti-freeze. The studs just close the exhaust-gas openings from the cylinders. If you remove the exhaust flange at the aft end of the manifold (this is the part the exhaust pipe/risers threads into) antifreeze will come gushing out. It is the exhaust flange that seals the aft end in the manifold.
Here is what makes the project time consuming: Once the manifold is removed, you've got to decide what other work you're going to do while you're in there (unless you foresee yourself repeating this job in the future). 1) Along with the manifold, you'll likely be removing the attached exhaust elbow as well--might as well take them both in to a radiator shop to have them boiled out. 2) May as well take the heat exchanger in for cleaning too since all your anti-freeze is drained and half the hoses are already removed. 3) You'll never find a better time to replace all your engine hoses. 4) Ditto for your starter and fresh- and raw-water pumps if any of those look or act old and worn. 5) Ever thought of degreasing/repainting any part of your engine? 6) If you have a plethora of ground wires attached to various bolts on the manifold and exhaust flange, now is a good time to re-route them to a ground bus-bar, too.
I'm pretty sure the alternator bracket kit comes with the three exhaust manifold gaskets you'll need when you reinstall the manifold. If you separate the exhaust riser flange from the manifold, you'll need a new exhaust flange gasket too.
Here are a couple of blog posts I made after re-doing my alternator bracket and exhaust elbow. Hit me up any time if you have more questions.
The beginnings of this project came out of this post: http://c34.org/bbs/index.php?topic=5078.0 (a Catalina 34 forum, but applicable to Universal M25/25XP engines) This post helped me realized that my hoped-for, turn-key, Ericson...
ericsonyachts.org
As I mentioned in the previous post, I was unable to remove the bottom flange nut that held the exhaust flange on to the exhaust manifold. It wasn’t simply a case of the nut being frozen on the stud; there just wasn’t enough physical clearance...
ericsonyachts.org