It is actually a hydraulic fitting size. Might be able to track down one local. I tracked down a cap for the EGR port on my Thorley headers when I converted to the Marine intake. Been a while but...

Honestly I did a different approach on the last Vortec I built for a carb application. I have had numerous OEM Vortec blocks that had the bypass hole in the block cast and machined into it. Only the...