Steve, there are quite a few differences between kings and monocleds, though that shouldn't come as any surprise. First off there is size. Monocleds tend to max out at around 5 to 6', whereas the record length on a king is somewhere over 18'! That's definitely something you'd want to keep in mind. For this reason, the king will need a fairly large enclosure, and when handling the snake the utmost care would have to be taken. By the time it reaches 9', the good old hook and tail method is basically impossible to do without putting yourself in grave danger, therefore a trap box would have to be utilized.
Next there are dietary differences. Monocleds thrive on rodents. Most monocleds will eat mice and rats in captivity like they are going out of fashion. Kings, however, survive in the wild solely on a diet of other snakes. For this reason, you'll have to have a selection of good feeder snakes on hand that the king is willing to accept. Some kings eat any snake, some will only take ratsnakes from their area of the planet (radiata, Ptyas sp, etc). The issue of feeding kings rodents is a touchy one. Generally it involves a LOT of luck and a bit of trickery on the part of the keeper. I've read stories and heard accounts of people having to use catgut twine to sew rats and snakes together to try and get the king to take the rodent. Some people will take prekilled snakes and stuff them with rodents to try to ease the king over that way. Some people will blend snakes into a nice mush and cover the rodent with that to try and entice the king into eating it. Even if you get your king to eat rodents, you might get certain physiological problems. Rodents are more fat-dense that snakes (obviously) so fatty build up on internal organs can cause death pretty easily (this happens a lot in captive Eastern hognose snakes that people have switched from toads to rodents). That's not to say that it can't be done, as there are now a number of people with rodent-feeding kings that are doing great.
Lastly I'd like to address keeping the animals in general. I currently maintain 2.2 monocled cobras, 3 adults and 1 subadult albino. My snakes are handled very easily, they don't require very large housing units, and they are not too aggressive. However, they are the completely opposite of an angry adult king. Though I've never worked with O. hannah personally, seeing how impressive my monocleds can be and how intimidating they can be, I can only imagine what a large adult king would be like. That is one species that keepers always have to be on their toes around and never for a minute become complacent. Monocleds are much more forgiving than kings as far as survivability goes as well. You said you are getting a young king, which is good because it should be in good shape. Lots of w.c. kings come in in horrendous conditions, so heavily infested with parasites that lots die pretty quickly. Speaking of parasites, that's another thing I meant to address. If you feed your king snakes, no matter if they are alive or f/t, it would be a good idea to do a fecal on the king periodically to assess its parasitic load. Any time it looks like it's getting more than it normally should have, it would be a good idea to treat the snake accordingly. For this, you might have to find or already have a vet that is willing to work with such animals, or at least have one that will give you the meds you can't get at the store.
Anyway, good luck with whatever you do. And as always, be safe!
-Chance