Easier said than done. I suspect you would have to remove the manifold from the engine to remove the heat riser shaft. The valve or flapper could be rusted solid to the shaft. BTW, the orientation of the valve in the picture may not be correct so don't use it as a reference.
The over-sized shaft has this note: NOTE! This item is not available now, however we are accepting names to add to a list to be notified when we can get or produce this item. My take on this is that they need several orders before having some made. I would call and ask.
I wasn't able to find the bushings in the Chevy Master Parts List but with a little research on the Internet, you should be able to find what you need from a bushing supplier.
When the valve is closed, I suspect you have exhaust backpressure hence the most noise. Is the noise that bad when the valve is open?
If this is a car/truck residing in the warm state of California, you may not need a heated manifold. A person could fix the valve to remain open and pack the gap around the shaft. Just a thought.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
