The way I've introduced dogs of large size difference in the past(German Shepherd and Beagle) was to get on neutral ground which neither have been to with both on leash, and have two people walk the dogs up to each-other so they can do some sniffing. If one of the dogs isn't friendly correct them and walk away for a few minutes then try again.
Keep in mind that a ton of Chihuahua's are aggressive little bastards, so make sure to get it all the socialization you possibly can. Once they're in the same pack its safe from the pit, but if the chihuahua were to attack another dog the pit-bull might defend it, and then you've got a real problem on your hands.
*edit* Why on earth did someone vote this down?