The Streetmaster intake starts losing out at the top end when you approach 500 HP. For example, on my 390 Stroker dyno mule the peak HP of the unported Streetmaster was 472; with port matching and plenum work it went to 484. The Performer RPM unported was 493, and port matched it was 504. But both manifolds peaked in power around 5500-5800 RPM. The Ford tunnel wedge intake, which made 524 HP on this engine, also peaked in HP at 5800 RPM. The point is that the intake manifold will not play a significant role in the RPM where the engine makes peak power, except in extremely mismatched combinations. So if you thing the issue you are having is related to the engine not making power past 5400, I wouldn't be looking at the intake. The cam is the primary reason why the engine is peaking in power where it is.
You said you've tried three different cams? What were the specs, and how many cubes was the engine in each case? If you went up in cam duration, but increased cubic inches at the same time, you may not have moved the peak power point. With the same cam, more cubic inches will lower the peak horsepower RPM.
Having said that, there is probably power to be had with a different intake setup. Around the 500 HP level the multiple carb intakes start doing better than the single four intakes, so if you are thinking about changing the intake setup I'd give some consideration to a 2X4 intake, either a tunnel wedge or a medium riser. You could also try a Victor with a Dominator carb, which has much the same effect as multiple carburetion because of the size of the carb.