Edelbrock valve spring replacements

As explained in my PM, the unported heads, dual-plane manifold and 1 5/8" headers will be holding you back. 5600rpm is about right for similar 408 combos. You won't see any more than 6500rpm until you work those heads and get the engine to breath better. But then, you don't really need to see 7000rpm to make 500+hp with a stroker motor - 6000-6300rpm peak power is about what you should aim for if you want a serious street-strip effort 408.

While Edelbrock says the Air Gap is good for 6500rpm, that's on smaller 340/360 engines - you really need a single-plane manifold like an M1.

As a basic rule, if a cam manufacturer describes a grind as, say, 2500-6500rpm for a 340/360, you can subtract about 300-400rpm of that for a stroker to give you an idea of where it should work.

There are better modern cams than the 557 purple shaft, but your present cam is not unsuited to your present combo - you wouldn't want to go much bigger. You need to look at the whole package before deciding on a cam upgrade.