[PontiacsForLife]

I suggest getting an exhaust manifold without the butterfly valve, and install it in place of the factory manifold. The heat tube would be removed, and the hole under the carburetor would be blocked off with a simple plate and high-temp gasket.

Of course, there are several opportunities to run into stubborn bolts: Those 6 that go into the head, and the two that are under the intake.

I haven't answered your question about how to effectively remove the exhaust manifold bolts, but I do have a suggestion - despite my suspicion that you already know this tip, looking at your herd of cars: Bolts loosen up when there is a sudden torque applied - not a gradual torque. For example the whacking of an impact wrench is better than a constant torque with a breaker bar. Since we can't likely get an impact wrench on these bolts, maybe a hammer whack on a breaker bar.

The risk of breaking bolts, I feel, outweighs the potential for exhaust leaks under the hood due to the butterfly valve and the three extra exhaust connections you'd be eliminating.

...at least that's what I'm going to try...
-Adam