So before I replaced the water pump, a 4mm spacer had been fitted front & rear of the fan.
I think longer bolts had been fitted, as I bought the correct bolts from Minispares & now there is no where near the same thread length as before.
For now, I will leave the spacer off & assemble the radiator with cowling. Then check where the fan sits in relation to the cowling/radiator
Respectfully, don't work to the length bolts you have, get it all to fit up correctly, then if need be, get the correct length bolts to make that combination work.
The fan to radiator fact dimension is unimportant.
As I said earlier, the key to optimum flow is having the mean blade position in the centre of the lip on the shroud. Moving the fan face towards the radiator whilst the blade tips are not fully in line with the shroud return lip will reduce air-flow and thus reduce cooling.
The Mk.1 2-piece shroud has the lip facing the opposite way from the later cars. Just make sure the fan blade tips are running within the width if this lip.
I agree completely with what you've mentioned here in regards to fan efficiency and to add to that, keep in mind, right in the middle of the fan itself, is a dead section by way of the hub of the fan, which is much worse with the plastic fans than the metal ones. If the Fan is too close to the Radiator, there'll be very low (if any) air flow though that part which could end up being blocked off by the Fan Hub.
The other aspect, as I mentioned ^ is to be sure you can get a new fan belt in there.