Hi Tony;
I also had problems with my 0-300D running at those temps. Turned out
that the ignition timing was 10 degrees over advanced. I found this out
by accident when I had to replace a failing mag myself. Both were
overadvanced. My A&P sweares he checked them at the last annual. Note
that the correct timing mark is a pain to find so some people may be
tempted to pencil whip checking it(If I remember it is on the underside
of the crank when it is at TDC). The two mags are also timed to
different settings. When I reset the timing the engine ran exactly the
same just 100 degrees CHT cooler. Now I can't get it over 400F almost
no matter what I do. Full power cruise is about 375F. Note that the
advanced setting eventually raised hell with my rings and I had to
replace them later on. On the same engine, some time later, I had a
sucked intake gasket that made one of the cylinders run cold along with
a lousy idle.

Jim