As Red stated, the HP has a solid lifter cam and the bottom end is stronger. The block has heavy duty main caps and the crank has higher nodularity that a standard 289. The cyl heads have screw in studs instead of pressed in. What does all of this mean on identification? Not alot if the engine in still in the car and you can't take off a valve cover. They also came with a dual point distributor instead of a vaccum advance, but that can be changed out pretty easy. The block casting numbers are more than likely, your most accessible way to find out.
Unfortunately these were all used for other engines as well, so it's still a bit of a crap shoot. If it's out of the car there will be a swatch of orange paint behind the flywheel. I hope this is somehow helpful. As for horsepower build up, it is a better platform. In order to build horsepower in a small CID engine, higher RPM's are a necessity and the HP is built with this in mind.