The data displayed on the HP gauge has to be a calculated value based on vehicle weight & speed/time since there is no measuring equipment such as a dyno connected to the car or engine. It's a rough figure. I've had other cars on dynos but not the SP, but I understand different makes of dynos do give different results. IOW it's still a rough figure. Here on some online calculators which provide figures that approximate a 392 Charger's horsepower.
Engine Motor Horsepower Estimate From Elapsed Time Calculator
Engine Horsepower Calculator From Elapsed Time: Quarter Mile Drag Race
Engine Motor Horsepower Estimate From Trap Speed Calculator
Engine Horsepower Calculator From Trap Speed: Quarter Mile Drag Race Speed
This one calculates flywheel HP and wheel HP, and it's a 10% loss according to their figures.
Horsepower Calculator From 1/4 Mile Stats
Wallace Racing HP Calculator For 1/4 Mile