70duster 360 gets hotter the faster i go

I have a very similar combo.Its impossible to overheat,mine. You have one problem. maybe two.The first is insufficient fresh airflow through the rad.The rad is unable to shed the heat. The second is the creation of the heat in the first place.
At cruising speed,say 2600rpm your engine wants in excess of 44* advance,probably closer to 50*.So if you dont have a working vacuum advance unit, thats the first thing you will need.-----------------Street cars need V-cans.Period.---------------- Once you get it on there you will likely need to slow the mechanical down to all in by 3200ish.
-When cruising,if you dont start the fire early enough, the fuel continues to burn while the piston is traveling down the cylinder. This means the cooling system is exposed to all that heat.And the loss of pressure at the top translates into less work performed and therefore more than necessary throttle opening.It may even be still burning as the piston comes up on the exhaust stroke. That means the piston has to push the hot expanding gasses out. Thats extra work for it and ergo more heat.Late cam timing can give a similar problem.
To see if Im on the right track,do this;A) remove the oil coolers that are in front of the rad and roadtest.B) remove one of the fans,and roadtest. C) crank the timing up to 40* and roadtest.Be very aware of, too much throttle, inducing detonation.Put the timing back at the conclusion of the test. Conversely you could retard the timing 20* and see how much easier it is to overheat.
My money is on the timing making the biggest improvement.If it was me Id go straight to the timing.

Dont blow it up!!!