I fill radiator up 3/4 and allow to run and get hot. Meanwhile, front end is jacked off ground 6-8" so the cooling system can burp air pockets, then I fill slowly so all pockets bleed out. Fan placement is critical.

Only thing I can think of is radiator sealing to cowl and shroud sealing to radiator.

I am running 210 with a 520 hp 455 with 4 core aluminum with dual puller fans. In traffic and 90 degrees she will get to 220. \

Lean carb jetting will have some impact, but not much. As well, I screwed with fan placement and changed it to where its only 3" inside the shroud. That got me the best cooling.

Water wetter is snake oil and a band aid.

Is your timing advance adjustable or pinned at the distributor?