I believe it was mentioned here, but just go ahead and tap that nipple for 1/8" NPT and put a real pipe plug in it. Then use the dorman cap just for the aesthetics of a finished edge. I ran that on my 318 for at least 7 years.
There is no nipple on the manifold for a return that I can see. There is just that boss where an AC compressor would mount. My M1 intake has that extra nipple where I could feed it back to the manifold.
I'm talking about tapping the ID of the water pump heater hose nipple to 1/8" NPT. The one you put a rubber cork and all-thread on.
