I don't think there is any problem with putting the later fan on an earlier car, only that the later fan is supposed to be virtually impossible to find. The few I've seen on Ebay and such have been missing a blade or something like that, and they still wanted quite a bit for them.
I'm planning an upgrade to an electric fan this spring myself. In my opinion, if your car is not a show car then an upgrade to an electric fan is a wise choice, especially if you live in a hot climate. I have made several upgrades to my car in the name of performance/reliability in the last two years, and this is a good one. In the vintage Mustang community they call this "resto-mod", I'm sure they call it this or something similar in the LBC community as well. So far I've done an upgrade to the Pertronix ignitor and matched coil, very nice mod. Not having to play with points has been a godsend as I hate them. I put the ignitor in almost two years ago and haven't touched it since. I've also done an electric fuel pump, if anyone wants some ideas on that I can provide them in another thread as I went a few different ways before coming on the present setup. I have part numbers and sources for that stuff.
Anyway, to the cooling fan specifically. I'm going to go with something similar to what lastdeadlast is talking about, a fan large enough that will bolt directly to the rad perimeter, in puller orientation. Get the biggest fan you can that will physically fit over the rad and between the rad and the crossmember, and not overload the cars electrical system. Places like Summit racing or any good hotrod shop will have lots of choices, with specs. I will remove the factory fan altogether, as I have heard also that it is immensly quiter under the hood without it and it really doesn't work that well anyway. Also, there is a BMW thermostatic switch that will work well, and a fitting for it can be brazed into the radiator or a holder can be made to fit into one of the spliced rad hoses. This is a 180 degree t-stat switch if I remember correctly, and will kick the fan on and off automatically. Additionally, there is a three position switch in the cockpit for manual override, on, off, and auto. I have an article that someone wrote up with this very setup (where I got the idea), with part numbers and wiring schematic for everything, if anyone is interested I can dig it up.