Blown head gasket, likely cause????

I think most of us are in agreement that is detonation.
Double check piston tdc to mark on crank. Did you re-curve you dist? The reason i ask is that you may have set the timing at idle, and not realized that there was to much mechanical advance in the dist causing it to be over advanced. So when your dad was having fun with the long skinny peddle, it when into detonation. :burnout::violent1: