Couple things to consider.
With a 400 do you have the lower valance in place? Do you have the upper filler panels installed? These are 2 things that many people removed at sometime and never reinstalled. Very important to have these in place for proper cooling. Another item is the water pump, many times the water pump was replaced but installer didn’t pay attention and may have not installed a high vane pump like what was installed by the manufacturer. The high vane water pump helps to reduce cavitation. Cavitation causes high temps do to poor coolant flow.
Have u tried a high flo tstat? This helps considerably too. Also check if your radiator is 4 core if it’s a copper radiator and has sufficient flow.

I bought my 69 in 1970 and have had issues u described, plus severe pinging when starting out and going up hills, temps would get up to 215 or higher on the gauge at low speed, over the years I have tried about everything with no significant help. I have the. Lower valance, upper filler plates, high vane pump. Last year the radiator had white spots showing signs of leaking so I replaced the radiator with a 4 core copper from US Radiator, installed a high flo 180 degree tstat.
This totally illuminated all my issues.