All chassis dyno give you the net reading (true HP to the rear wheels), not the gross reading like the factory uses on there adds to the flywheel.
My last pump gas street car had a 517 C.I. 400 block stroker motor that made 612 HP at 5500 RPM and 644 Ft. Lbs. torque at 4500 RPM with a set of ported 906 heads with big valves with 9.25 to 1 compression ratio and big solid lifter roller cam on a DTS brand engine dyno with a low deck six pack set up tune to the max, that motor made right at 500 HP net at 5000 RPM peak with a 10 inch custom converter on a local chassis dyno, that is 19.4 % loss with 4.10 gears in the rear end
On your deal you may have more power in the engine that is not getting to the rear wheels because it is running out of fuel above 5000 RPM, been there done that
If I was you I would make sure that the motor is not going lean above 4500 RPM at wide open throttle in high gear, look at buying and using a good wide ban O2 exhaust system so you can see real time air fuel ration while driving