A few more thoughts:
Perhaps the better question is to ask: is it typical for the 350/400/455 to run more than 5-10 degrees above the thermostat's rated temperature? And if so, why?
Coming from the world of '55-57 Chevys with first-generation small blocks (265 and 283), if it's running over 190 degrees with a 180 thermostat, something's wrong. (My experience was more like 180-185, even on 90+ degree days.) And, these cars didn't even come with a fan shroud or fan clutch. So, I'm trying to understand the differences here.
Digging around, I found a copy of the 1969 Pontiac owner's manual online. The scans included the emissions equipment folder as well. See:
https://oldcarbrochures.com/static/N.../dirindex.html and
https://oldcarbrochures.com/static/N...nsert%20a.html
Here is a summary of the exhaust emissions changes to control HC and CO (NOx control was not yet a requirement in 1969)
https://oldcarbrochures.com/static/N...nsert%20c.html
Based on this page (and some independent checking) the '69 Pontiac came with a 195 degree thermostat.
So, with a 195 degree thermostat, I would expect 195-205 degrees. OTOH, if you used a 180 degree thermostat, I would expect 180-190. But, I'm gathering that often isn't the case?
A random thought - I wonder if folks have problems with the fan clutch not properly freewheeling when you're at speed? A fan clutch that properly locks up at idle but won't freewheel at 30-40 mph can actually interfere with proper airflow through the radiator. (This would be one advantage to an electric fan - it won't interfere with airflow at speed.)