wrench87,
As usual, "it depends."
An
oil-filled coil mounted horizontally atop the engine will fail. Now, if it's on a show car that will be driven a thousand miles or so in its lifetime, no problem.
My MSD Blaster II was mounted horizontally and left me in the most rural part of Florida I've seen - very beautiful but a long way to go get the trailer - at about 14,000 miles when it failed.
I bought an epoxy filled MSD Blaster II and so far, so good at 24,000 miles.
Electrical interference is real. Try the routing-wires-together layout and let us know what does or does not happen
Tom