[brucepick]

The highest engine temp I ever achieved using the block heater, 105 deg F, was after I insulated the valve cover.

Also,
Today I ran the heater with some snow on the hood. First time warming the engine following a recent snowstorm. The snow melted away first in a patch on the passenger side of the hood. That's where the distributor is attached to the head. Engine itself is on the other side of bay, where the hood was still covered with frost and snow after the heater ran about 8 hours. The heat was conducted into the head where it escaped to warm the hood. Heat did not escape from valve cover apparently.