How far to rev this 273 commando build

HP is not just a function of torque . It a a function of torque and speed (rpm).
A steam engine can produce full torque immediately (0 rpm). 0 hp.
There is nothing magical about 5252, its just a number that results because of the units of measurement that are used. There is no way to calculate hp directly from torque.
Of course a dyno measures horsepower. It measures torque and rpm at every point in time and reads out horsepower and torque.
Doesn't an ammeter measure amps? It actually measures a voltage drop across a resistance.