I have used plugs as thick as the one shown above, and I have used thinner plugs. Depends on the location on the block, and it depends on the source of the plug. These are not the right sizes but it shows the variety of designs that are vailable. I have seen many designs of plugs used on 216/235/261 engines.

Clean the hole in the block with a wire brush (on a drill or dremel tool). Some people use no sealant, and some people suggest using sealant. I have installed the plugs with and without sealant. I prefer using sealant.

From Chevs of the 40s - soft plug set - block casting plugs, expansion plugs, etc

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.