Having had a car with a low buck RB, and another with a W5 416, (both high 9sec combo's) starting from ground zero you will have a cheaper less stressed combo from the big block. That's a no brainer.... but since you already have the 408 (not sure of the combo) you might be ahead of the game to stick to it and make some improvements. Going big block will also end up with a trans change, and more than likely a new torque converter purchase which will factor into the cost of the build.

Our big block is gone... and we're sticking with the small block. Not sure why....but we prefer it that way