A idea that I've contemplated for many years, is to use a large disk brake, connected to a drive shaft, on the bolt pattern. Mount the caliper, to a lever, 1 ft long, to a point (say a bolt) from the center of the disk. I would then mount a 1 1/8 wheel cylinder, under the bolt. The wheel cylinder should be honed to 1.1284, for a accurate 1 square inch. That way, your torque will read, directly from a standard pressure gauge.
Run the brake line to a master cylinder, mounted on your panel, with a lever, next to your tach.
You can load your engine for braken and then do HP runs. If the disk over heats, run a water spray, into the center of it, to cool.
The formula for HP, with a 1 ft lever is, Torque X RPM divided by 5252.
Take your max tq reading, then record in 500 rpm increments and do the math for max HP.