You need a 4-way, not a 3-way. As Shockers are a perfectly balanced spool, you can't even use constant air as an air spring and cheat with a 3-way. Needs shifting air to each side of the spool.
I've actually had significantly worse performance with the pneumadyne 3-way in mechanical ion conversions than the SMAV-3, but I could have gotten a few lemons. They claim higher flow on paper, but making no changes other than the valve I get a significant velocity drop with the pneumadyne.
I wouldn't tap the body for barbs, I would make a manifold that is tapped for barbs that way you can readily convert the gun back to electro if desired, and you don't risk destroying the body as they have a reputation for developing stripped holes over time as is from the grade of aluminum used.
You may be best off with one of the cocker 4-ways that can handle full pressure. I haven't found an overly desirable mechanical 4-way that is rated for that kind of pressure.