I did exactly what Joules just mentioned; an electric booster at the outlet of the rad., combined with a mechanical pump. I used a Stewart electric pump because if they fail, they fail "open". I never thought it was possible for a Pantera's cooling system to work so well. I did this on the advice of Stewart Components, who make very high performance mechanical water pumps for NASCAR, etc. They thought, pumping coolant through the Pantera's 20' of tubing was too much to ask of any mechanical water pump. They thought it must cavitate badly at higher RPMs. Their suggested solution was an electric booster pump. They also said to remove as much restriction to flow as possible. Interestingly, they advised against multi-pass radiators, as they created too much restriction.
Peter, if you are determined to go the electric pump route, you might consider using two of them. One at the motor and a second at the outlet of the rad. Speak to the manufacturer first. A second pump might be completely unnecessary. I have heard it's best to remove the thermostat when using an electric water pump. The PWM controller simply runs the pump at it's slowest speed until the engine reaches normal operating temperature. From there, it speeds up or slows down the pump to maintain a stable coolant temperature. That's the theory anyway! If you were to do that, with a 351C, you'd need to block off the bypass.