I have similar problems with an SBC 350 in my '75 GMC, so I've been reading through various articles and troubleshooting guides.
Most troubleshooting guides says that high speed cooling problems are usually due to insufficient radiator capacity (either too small, or plugged up), or insufficient water flow. The guides also say that some type of air flow limitation is also a possible source, either due to air damming up in the engine compartment, or missing air dams/skirts that let air go around the radiator.
For your BBC, checking the lower hose and pump for proper operation, and then switching to a four row radiator is probably the logical first step, followed by a switch to a high flow water pump. The suggestion about removing the hood seal or propping it up at the back to allow more airflow is also a good idea, since its very cheap to try.