that relay will work.
Are you installing an air horn that requires a tank and compressor, or just the kind you can buy at the autoparts stores that come with the little compressor that just puts out air when you need the horn?
The smaller kind that does not require an on-board air system will work fine with the stock relay, infact I currently have that in my truck, and it works, but I think that horn is worse than the stock one...its louder, but the tone is more girly.
If you are running a horn that requires an on-board air system, you really dont even need a relay, but if you hook it up to the stock relay it will work just fine. I just installed a system like this over the weekend using a 5gal air tank and a small compressor from PepBoys. It works great using the stock relay. I have yet to mount the horns, but once I get that done I will post pics.