1967 318 Barracuda running hot...

Overheating at speed usually means the rad can't shed heat fast enough.
Either the air-stream is not getting to it, or thru it, or out from under the hood.Or
the heat transfer from the fluid to the metal to the fins sucks.Or
you have a crappy heat-transfer medium.Or
you are stuffing way more heat into the rad than it was designed for.

So;
First question is; is the rad sized correctly for the engine?Are the rad fins still attached to the tubes?
Second is; what coolant are you using and at what percent water?
Third; does the lower rad hose have the anti-collapse hose in it?
Does the car have A/C?
Is it an automatic? What stall? Auxilliary cooler in front of the rad? Any trans issues?
Headers or logs?With free-flowing exhaust?
What rpm are you cruising at? With how much total timing at that rpm?
And then;how much power is that bad-boy 318 putting out; or rather what size cam are you running?Was it installed correctly?Iron heads? Do you know your cylinder pressure?
You still got that front license plate mount on there?
Are there any chassis issues like dragging brakes and severely under-inflated tires or dry U-joints ,or does it needs an alignment bad?
And like somebody said; is the waterpump turning?And in the right direction?
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I have never in 50 years needed to burp an SBM.
I have a dial-back timing device on my dashboard. It has a range of 15*. My aluminum headed,10.9Scr,367 cuber,cares not a whit about cruise timing as it may pertain to the temperature gauge. So while retarded timing can send heat into the exhaust, that's for sure, my temp gauge never sees it.
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The heater has nothing to do with an engine overheating issue. Running it in reverse-flow only traps air and makes it noisy,usually just for a while.