A large single will be fine, of around 14", remember the only time it will be needed is when there is insufficient natural air flow to cool, so speeds are low, and hence heat isn't massive.
The curved vane Kenlowe one is the kiddie, the std fans are TOTALLY adequate though if working ok, a simple fan rewire is a lot cheaper than new ones, yes the fans do consume more power than newer curved vane sorts, but you won't notice any power difference with the std fans vs newer ones, std fans were just picked out of the ford parts bin, but do the job, I certainly won't be changing, unless they die, then I'd look around the scrappies are curved vane are pretty std these days.
Don't be fooled into buying a cool running stat either it is a gimmick, that offers no protection whatsoever, heat doesn't kill HG boost does, I set my ecu for the fans to come on at 105C and go off at 102C, didn't bother it, I have a 88C stat, and currently set at 95C on 92C off.
tabetha