do the water soap around the plug while it idles,if it leaks it will bubble around the plug.

never saw a machine shop change the stock SB plug hole. even the new magnum heads use the same seal,the 5/8 hex is all that changed.

are the air injection holes on the heads blocked off,or are they non emission J heads that were not drill for air injection? with headers they are not coved and sealed by gasket. the need to be filled or blocked.

I dont see anything different on yours that my 88 360 heads does not have. same taper,same hole,same long reach plug. mine ar on the bench so I can see them very well.

try another plug like an autolite 65 in the hole,the NGK could have different plug body specs.

compair the 2 plugs together and see if they look the same.

I dont see any taper plugs crossing over to a SB myself.hard enuff to get the peanut plugs for my BB,they all stock the 13/16 short reach plug.