timing sounds good,may be as simple as a bad or loose fan clutch.
this http://www.4secondsflat.com/Spark_plug_reading.html may be worth a look. you didn't mention if you had a hood seal,may help directing more air in the rad. it dose sound like an air movement issue.I still have a slight issue on cruise days when traffic is still and temps are up,considering a elec pusher for that.I've noticed a slightly higher idle speed helps.in gear I'm at 700 but in neutral up close to 1000 and it keeps that gauge from bumping up to high. I run a Hayden clutch,hope this helps.