Electric fan won't cool at highway speeds

I bet there is an undersized or clogged radiator behind all this.

The rule of thumb is that overheating at idle is nearly always airflow related, while overheating at highway speed is a capacity problem. You seem to have some of both.

What radiator is in there?

Even with stock smallblocks I've found that a 26" radiator and deep-pitch thermostatic clutch fan was the only combo that would reliably stay cool.

With the AC condenser and trans cooler both in front of the radiator it's probably simply being overtaxed. The solution may be to upgrade the radiator and install a small pusher fan in front. That's not uncommon even with an engine-driven fan if the engine bay is too tight to move hot air out.

Everybody and I mean EVERYBODY plays the 160 degree thermostat card first because it's much cheaper than a radiator but that's just wasted money and effort. The thermostat is open at anything above it's rated temperature and if the temp keeps going up past that you have problems ELSEWHERE.