Personally, I would not plug the hole. Terminator: They make a round chrome "plug" just for this purpose, you can get it from Performance Parts. The cars with the star on the grille (SECs for example) look like it should be there, and the cars with it on the hood, look like it should be there too. In my opinion, cars with no star on the hood (on cars on which it came), look like there is something just missing....which something would be missing!
Enough on that. Yes the later ones have dual fans. I suppose you could retrofit one. However, that bar that has the horn on it has been relocated on those models. Since I am sure that bar is welded on, it would be an operation not worthwhile. Is your car overheating? Is there something wrong with it? You really would not notice a thing if you spent lots of time and money on doing this. If you want to do anything with fans, you should consider replacing your visco fan with electric fans. Then you might notice a difference. However, there has been lots of discussion on this, opinions going both ways of course. There is also something about replacing your thermostat with a negative one. Search this topic and you'll find a lot. Especially the post named "Electric Fans & Radiators". There's my humble opinion