My instruction manual on my blaster said nothing about horizontal mounting, tons of us are running them that way, just make sure your + & - posts are horizontal too.
"It is recommended to mount the PN 8202 and 8223 coils in an upright
position. The High Vibration Coil, PN 8222, can be mounted in any position due to its epoxy potting
compound."
I don't understand your comment in regards to the primary terminals as they are in a fixed position