RPM potential is primarily a matter of valve train control, air flow, and how much money do you want to spend........make no mistake about it RPM costs money. As I stated earlier, sm blk chevy have had decades of development work, parts are inexpensive, there is LOTS of information. 383/400 you could get into the 8-9000 range with out too much problem, past that point would be much more difficult, question is WHY? Power band at that point is very narrow, it would be like a light switch, "on" or "off" .