You can drill and tap into the air horn. There is also a plug on the intake manifold that can be removed - it's in different locations depending on 12V/24V, and in the case of 24V heads needs to be reduced (think it's a 3/4").
The best bet is to get a boost bolt - call Genos Garage at 1-800-755-1715. It replaces an intake manifold bolt and allows boost pressure to be read from the last point in the path before the cylinders.