Agree it sounds like a airflow limitation at idle. Suggest you get an aftermarket temp gauge, even if only as a diagnostic to see how bad it is or isn't. Up to 205 deg or so is good. Up to 220 OK and 230 or higher cause for concern. It probably won't boil until 250+ degrees with anti freeze and a pressure cap. I don't worry about creeping at idle if it doesn't exceed above limits as long as it cools on a roll.

In general, the MP fan kit used the smallest fan and loosest clutch to cool with minimum HP loss. If that's not enough cooling, a tighter clutch and/or bigger fan could help. I have had good luck with Hayden clutches. Standard duty for the 5 blade fan and heavy duty for a bigger fan (eg 7 blade from a late 70's Cordoba).

I'm not a fan of fixed fans (pun intended). At speed it sounds like you are driving an airboat. Lots of noise.

I think its worth checking how serious your problem is first and trying to match up a more capable thermal clutch and fan if needed.

FWIW mine does creep up ('70E, BB, AC, big radiator, MP thermal fan kit) but has never overheated... including summertime rush hour around Las Vegas. If desert rush hour was the norm, I would probably install my backup 7 blade fan and heavy duty Hayden clutch but in my application, no need.