Some years back when I was making & selling the spacers, 3/16" worked for the HS rockers & Eddy and Stealth heads I was using.
I bonded the spacers to the heads with #2 Permatex & used cork gaskets under the OEM covers, no leaks anywhere.
Painted the heads, spacers, aluminum intake & water pump stuff, and especially with the Stealths, it all looked stock to 90% of people.