You'll still save a little cash over buying a kit, especially if you already have the crank. A forged factory crank should be plenty strong for a street-strip motor in the 550 HP range.

Advantage over a 440 block is weight! The 451 combo uses shorter, lighter pistons, and the crank itself gets lightened when you turn the counterweights down. This lighter rotating assembly will rev faster than a tall deck assembly. The block itself is also lighter, so the car gets to shed a few pounds right where you want, over the nose. Fit also comes into play, especially If it's in an A-Body. Lastly, there's the argument that the lighter weight rotating assembly taxes the rods less when pressed, theoretically making it stronger, and its widely accepted that the low-deck block is a stronger at the webbing, because of the lower presence of the cylinder bores. thus the low-deck motor is viewed as more durable.

With all of this said, while I'd rather build a low deck, I wouldn't hesitate to build an RB motor either. It's a choice between good and really good.