Harry, just to clarify, you are looking for 550-600 rear wheel HP, or 550-600 flywheel HP? Brent is about right with a 15% loss through the drivetrain, so if you were 415 HP at the rear wheels you are probably around 490 HP at the flywheel.
At over 500 HP the Streetmaster tends to fall off a little, so I think one place for improvement would be the intake. I'd be looking at a Performer RPM or a Victor. But I think your biggest area for improvement is the cam, if I read into your post correctly that it is a flat tappet cam. If that is the case I think the cam is holding back your existing combination, so I'd be looking at a strong hydraulic roller, or mechanical roller, to get you up to 600+ flywheel horsepower. The heads are going to easily support that power level, in my opinion.
One other thing. Eagle cranks scare me. I was talking to the local crankshaft outfit's owner the other day, and was asking him what he thought about the aftermarket cranks that are available. He strongly urges his customers to steer clear of Eagle cranks and rods. He told me a story about grinding Eagle cranks. When you grind a crankshaft you get sparks coming off the grinding wheel. He says that on Eagle cranks, much of the time the color of the sparks changes as the grinding wheel moves across the journal. He said that indicates variations in the composition of the crank material, which of course isn't good. If you tear your engine down and the crank needs work, my advice is to buy a new Scat crank instead of trying to re-use the Eagle crank. You might even think about going up to the 4.375" stroke. More cubes equals more power.
By the way, great to finally meet you in person at the FE Reunion, and good luck with the rebuild - Jay