John, It's all about the heads, bro. Velocity in the heads, and chamber size/configuration, plus valve sizing (bigger is not always better), is what gives you the power all the way accross the rpm range. These motors make good power (TQ) down low, but the heads just can't allow enough flow when you get above a given rpm, depending on the cam. The problem with these motors is the heads, if you want to release the full potential of the engine. You can make it a lot better with just cams, but to do it up right, the heads need to be changed, or else weld up your 103/110 heads and start over with the machine work if you want to keep the badges.
Every competent builder I've talked to in the past few months has said the same thing...if you want these motors to run at maxium efficiency, bite the bullet and do the heads. Quicker machine, better fuel milage, faster revving, and COOLER running.