I TOO tapped the intake horn on the advice of members of the TDR. I took it off and drilled and tapped it. It's easy to do.
I used the drill size recommended on the tap, just use caution on useing a pipe tap due to the threads being tapered. Pipe taps are not like regular taps then get bigger the further you screw the tap in.
This is what I did. I drilled the hole and threaded the tap until I could get 3 or 4 threads (on a pipe plug) sticking through into the air horn. I used a pipe plug bought at the hardware store for this so I would not mess up my fitting.
Hope this helps.
See Ya
Chris