If is not the fan or shroud..on the highway there is far more air than any fan can supply...hell it will run without them fine.
high flow pump and NO thermostat;
The issue is not so much no thermstat but rather lacking the measured restriction of the thermostat hole... do not get any old 180 from the spares shop, get the std GM speced on or the GM high flow speced for your engine.
Rates of opening , how much opens, the diam of the hole all vary as to the application they are for.
You have classic symtoms of no thermostat.
The other issue could be the bottom hose re enforcing ring inside has corroded and collapsed, which causes the bottom hose to suck in and partly restrict the coolant...good possibilty considering your final ratios.. highway cruise rpms.
The other issue could be the radiator need cleaning...not flushing, but cores manually cleaned..
do you think it's time for an aluminum radiator?
Nope just routine maintance and set up like the factory intended....
The factory cooling system back then was WAY WAY over built for the application...hence why they run for yrs cores blocked up with no issues...till they get really blocked up.
you can spend a lot of money getting a Al radiator install etc, or get yours serviced at a fraction of the cost...its your money.