The Davies Craig pump (bottom right below) works fine alongside the existing belt driven pump.
All you have to do is to fit a simple thermostatic on/off switch (top left below) to make sure that the electric pump doesn't start up until the thermostat is open.
I used one on mine and it increased the cooling efficiency markedly. It never overheated even though I was using a Laminova oil/water intercooler (centre below) so the heat from the engine oil was also being transferred to the cooing system.
I wired mine (through a relay) so that it was constantly live which also prevented heat soak problems. The pump would run for a minute or so after the engine was off keeping the water circulating through the radiator.