Who built the motor and what do they recommend? First thing I see is 500 hp and 408 cubes. That tells me you are going to spin this motor up some.
Not being familiar with small blocks, my first question would be what oil viscosity you are going to run. Anything higher than stock, combined with lose bearing clearances is going to require more capacity. Actually either will change the situation, engine clearances being the bigger problem. It would be great if you could have some capacity added by adding a kick out on the pan if you have room. Skid plates could be used to protect a deeper pan if you decide you need more clearance.