Water Pump

[AndrewA]

Pottering around locally, temperature headed northwards and 3 litres of coolant vanished!
After investigating all usual suspects there's a steady stream of small air bubbles in expansion tank so I suspect head gasket. While it's a pain am trying to be philosophical ! It’s done next to no miles in 5 years and then 3000 shall we say spirited miles in 4 months so chances are something was always going to give.
Anyway - it's the Poly 318 engine and my question is this. Is the water pump interchangeable between all 318's? Usual confusion on the web and parts suppliers. Some say yes one fits all, some say yes but it's a pre 1970 type water pump I need. Can someone put me straight please?
Thanks
Andrew