I am in the process of fitting an 1108 fiat fire engine into my 500R (500 built on early 126 chassis. I have been working for the last couple of weeks on this very thing.
We are mounting the radiator up front on a subframe with a 10" electric fan. The pipes are routed under the car (there is a slight hump in the underside that is perfect for running pipes). we decided that flexible pipe would be best as it will take the odd knock and will not suffer from stress fractures.
The plan is to use the standard water pump and see how it copes, if it runs hot we will replace it with an electric one. I belive that rotarys run a lot hotter so an electric pump and bigger rad and fan/s might be needed.
Chris is posting a pic of the set-up so far

Pic added by C.Lakin