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The 4.0 that was used in the Mustangs was basically the same engine that was used in the Explorer and Ranger, it has a few minor differences like intake, and I don't think the Mustangs had the balance shaft in them. But like Ron said, it wasn't in the Mustang in 97.
It should also be noted that since the 4.0 SOHC is an interference engine the process of camming it requires aftermarket heads and thicker head gaskets.
If you have a pushrod version the Comp 411 and 422 cams are spoken of highly.
The pushrod 4.0 can be cammed without changing the heads, but I'm pretty sure they do have to come off.