Grand Prix 250
Main Motorcycle: '55 6T Thunderbird
Join Date: Aug 2008
Location: Chicago, IL
Just went out to measure mine, it's a '55 6T 650. The piston crown edge comes up to about .050" from the top of the cyl barrel, at TDC. So a little bit less than the .079" you are measuring. Not an exact measurement on my part since my calipers wont fit between the piston dome and the cyl wall, so I am sort of eyeballing it.
However, I have no idea if that is significant. Might affect your compression ratio.