A few years ago I wrote a Neapolitan Pet Health Care Insurance comparison, since that time the field has exploded!  When only a small handful of companies existed to now when new companies emerge regulary.  I highly recommend pet health care insurance for your neapolitan mastiff.  Giant breeds require larger doses of medication which cost more, giant breeds are charged higher costs for all surgeries, procedures and often just exams. 

We have used the AKC health care plan for a few years now and generally have been happy with it, the do have a large list of "do not covers" but their accident and injury portion of the policy is excellent.  Below are some links to assist you in locating a plan that suits you:

• Pet Insurance Review USA & Canada
• Pet Insurance FAQ's by About.com