The coolant/water will expand(about) 2" in the radiator from cold to hot.
I use an overflow tank for 2 reasons.
Just in case of over heat, and its required at all tracks.

Not sure if you posted.
Are you use'n a fan shroud? Makes a big diff.
And make shre it's close to 50/50 into shroud opening.

If the "clutch" is good, it's better then a fixed blade over all.
A 19" flex fan will cool best at highway speeds, but you should fix the problem 1st.

The fillers work best at idle, but also help at highway speeds.

Factory thermostats are 195*.
My 1st 68 FB 400 ran 210* with a 195* stat for 5 yrs(1975-80) without any fillers, so i dont think 230* would hurt.
I do like'm at 180-190* though.