Dave, the 50% rule is pretty useless if you think about it. In an engine there are components that are rated differently and your engine is only as strong as it's weakest part.
A good example is the stock rods. In a cheap OEM small block the nodular cast crank is capable of handling ~800hp. Cast pistons 600hp and PM rods 500hp. As you can see, rods are usually built the smallest, this actually decreases the wear and tear on the bearings because they are light weight. A heavier rotating assembly at high RPM will require more oil than a lighter assembly.
Now there is the other problem with engines... they're ability to put up with damaging forces like cheag gas, stupid operator (not me), too much timing, exhaust leaks, intake leaks, nitrous
. If you detonate with cast pistons you can kiss the piston tops good-bye, fordged will survive longer without failure.
A classic example of the above is the GN motors. It's a 60's engine with cast internals (including pistons but higher SI content) and they've seen 600hp on stock bottom ends. If you do the math it's pretty amazing.
This is why I don't believe in building a forged engine unless you've blown up your previous engines with poor tuning or are shooting for >600hp. Now that's just talking about a 350, a 383 and 400 have a longer stroke which puts more stress on the rotating assembly (higher velocities and accelerations). So for a long stroke motor you NEED to use high quality rods!!! I can't stress this enough. I've seen my share of wimpy rods in 383's spinning over 7000rpm and lossing a rod, it's almost as prevalant as the ford guys cracking their stock blocks.
So spray away with the nitrous but be sure you don't have too much timing or you'll blow it up.