High 400's for the horsepower with HP exhaust manifolds and mildly ported iron heads shouldn't be too hard to accomplish. It seems to be easier to get over 500 when you start putting some compression into it and you won't be pump gas friendly. With exhaust manifolds, you need a cam that has minimal overlap (unlike the 110 LSA on that cam in there). So if you want a choppy idle to sound cool, headers and cam overlap are your friend. If you are cool with a milder idle sound and making power with exhaust manifolds, you can get them to work pretty well. I'd recommend a Bob K "Mr Six Pack" cam or calling Dwayne Porter for something custom, Andy's testing articles he linked to above are good info too...