The chrome cover wont really help, its more of a dressed up safety item, if it doesnt surround the fan, it wont draw any more air.
Is the radiator up to the job, clean, no plug rows?
Is the cap holding its rated pressure?
Since its only on the highway that it heats up, Id look more at timing and jetting, since youre getting maximum air flow at the higher speed..
Does it cool down right away when you get off the highway? That would definitely be more indicative of a lean fuel mixture or retarded timing.
By the way, 220* is not overheating, most newer cars operate close to that temp, but getting it to run cooler wouldnt be a bad thing in your case.