Owners of P30 454 chassis, what are u setting the timing at?

Having been a GM Master Tech, all of the above reply's are good, and I would consider checking each one, BUT, there is another possibility. Some of the Mark IV engines had their timing marks below the harmonic balancer @ a 6 o'clock position, if you were to imagine the harmonic balancer as a clock face. If the installed engine is a crate motor, the timing tab may be incorrect for you're application, or the installer may have overlooked swapping the tab to it's proper position.