Crypt Ogres

I'm sure this has been covered before but would is the best way to convert a box of Bulls into Crypt Horrors?

Not sure I've ever seen this done. However - Gorgers would be a closer model in looks, and people have converted manga. Bull to a forger so maybe have a look for that in the threads search option and apply a different paint job I think you may have found a winner!

You could make them into big Ogre zombies?

Well, it's tricky, since Bulls are fat around the waist and Crypt Horrors are essentially lean killing machines. I'd suggest Belchfist's way of going about it, but if you're wanting to work with just the Bulls box...

You could try focusing on their poisoned attacks aspect, and have the Bull Horrors (Crypt Bulls?) have elongated arms with long, naily-clawed fingers, while having pustules dripping poison (not too Nurgly, but you can get a similar idea). Or, you could GS them and hulk them up, making their upper body larger proportionately to their guts, and making them BIG Horrors... I dunno, it depends on how much Greenstuff you plan on using.

The easier route would probably be Randroid's suggestion, if you wanna go with Bulls-to-Horrors conversions

I went with the idea that they were a form of zombie ogre that counted as crypt horrors rather than trying to make them look the same sort of shape. If you want the latter, then use the actual models. (I also have to admit I wanted something cheaper and more personal for my undead army.) See http://www.ogrestronghold.com/forum/index.php?topic=28319.msg370731#msg370731