Drilling the manifold boss will require you finding the correct offset bracket to mount the horn there. If it fits in the firewall location, then that would work fine.
I think there'd be plenty of room to drill the boss. As I mentioned you might need a slightly longer drill bit to keep the chuck from hitting the manifold. The attached pic shows a standard length 19/64" drill bit chucked all the way into the drill. You might be able to chuck the bit shallower and avoid hitting the manifold with the chuck.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
