Warm the cylinder head with two large electric soldering irons like plumbers use,
and chill the lower metal section of the spark plug with either a chunk dry ice,
or the drug store Wart removal spray chillers.

Make metal temperature differences your assistant.

I once saw in the 1970s an older field representative from the Roots Blower company use metal BB's during a bearing installation. He used both hot and cold BB's . He chilled his BB's with cold blow off gas from a high pressure cylinder of CO2 gas.