Turn the BBC mains down .100" and machine the snout to 409 specs. Reduce the counter weight diameter to clear the 09 block. You will need heavy metal to re-balance the whole assembly. BBC rods and a custom pin height piston.
My guess is that the boat guys want big torque at lower RPM's. The 409 isn't noted for real good air flow to support top end HP. With a 454 crank you would get a motor in the 474 inch range that would make stump pulling torque. Dan