408 breaking up

What cam is installed? A 268H isn't going to run up there anyways... a 295hl or 305H would.

Fuel system details? The one area where people tend to not have enough umph.

Where is the rev limiter set. If it's at or near the 6000 range, that could be your issue. Turn the limiter at least 700-800 above your desired shift point. They have a tendency to start pulling spark earlier than the actual set rev limit point.