I assume you're using the electronic temp reading from your EFI system, not the known-inaccurate stock water gauge. Where is its temp sender? It should be in the block under the thermostat to match where the stock sender was in millions of Mustangs, Fairlanes and Galaxies. High indicated running temp was one of the earliest 'problems' Ford encountered (1972!), and they used several no-cost-to-Ford fixes. They are all listed in the Ford TSBs (Tech Service Bulletins), of which you should have a copy.
One was putting the stock gauge sender in the small water reservoir to drop the indicated temp by 10-15 F. Another was to install a resistor in the gauge line to do the same thing- often combined with the odd sender location. And there were two different resistors recommended- a 5 watt and later a 10 watt!
Note there were also two stock gauges- a 0-220F and a later 0-250F. They read differently on the same engine. Normal 351-C running temp is 210-225F with excursions higher under heavy load. The early gauge will be pegged which scared owners so Ford went to the later gauge in mid-1973. As mentioned, my late model Z-28 normally runs 230F; 'Hot' is 250F!