Engine RPM will continue to climb so long as a few conditions are met. 1. Most obvious is that the engine does not suffer from catastrophic failure. This encompasses anything from broken connecting rods to a broken rocker arm to ......... whatever. 2. Valve float. This is pretty much going to dictate to itself where the engine RPM stops climbing. Once you lose control over the valves all heck can break loose. 3. Airflow through the motor. This encompasses the carburator, intake manifold, heads, camshaft profile, and exhaust. A really poor flowing set of heads will starve the engine of airflow and prevent the engine from turning over a certain RPM as will a short duration, low lift cam.
All this being said, you have to look at the combination of all three. A really good lightweight bottom end, high flowing intake track and a roller cam with high spring pressures will spin all kinds of RPM. Pull off those heads and stick a pair of stock 441's on there and it probably won't spin past 7200 even with adequate spring pressure.
or put that awesome top end together with stock rods with stock rod bolts and don't expect 9000RPM's. Expect to be pulling pieces of cylinder wall out of you teeth.
There's always more to it and I'm sure that others will add their 2 cents.
BSE Racing Engines