The correct horn for a 1927 is a Klaxon 8C that mounts on the exhaust.They can be bought on Ebay or from Earlychevyparts.com. Here are a few pictures of them.
Steve '25 Superior "K", '79 Corvette , '72 Corvette LT-1 & 1965 Corvette Coupe
The 1928 horn fits on the drivers side frame across from the oil fill tube. Here is a picture of the mounting bracket you will need. They cost about $40. The Klaxon 8C is correct for 1928 and '27. Also I found a picture on line of the horn location. I hope this helps. Good luck.
Steve '25 Superior "K", '79 Corvette , '72 Corvette LT-1 & 1965 Corvette Coupe
When I fitted a 1928 head to a 1926 car I made a new bracket like original to fit rear of 28 exhaust. Worked great, but here in Australia 28 exhaust manifolds are different so may not work on yours.