Since you put a limit on rpm, i would follow Andyf or Cabs' advice and go big. It depends on what you are willing to invest in the shortblock, but my preferance is go big, 512. This is the best situation that the statement YOU CAN'T BEAT CUBIC INCHES works in. If you have 25 percent more cubic inches than a 400, (512) you should be able to build 25 percent, or close, more torque way down low (2500 rpm?) where you say you want it. A custom set of pistons designed for the good heads down the road would be my choice. Then when you upgrade, the gains will be very well worth it.