Yes they did - but the factory didn't care if the coil crapped out whenever.
Being oil filled - when you lay it on it's side, the area at the top along the whole length of the coil is now exposed with no oil.
Vertically mounted, there is also a small amount exposed at the top, but the area of that vs the area along the length of the coil is much smaller.
Besides, on a small block it was placed at the back of the engine and the air cleaner covered a portion of it.
That's alot of heat soak when a hot engine is turned off vs it sitting in the engine bay with more air around it.
My preference is to mount them vertically and get them off the engine. Your preference may differ...
I did not have to to anything to my wiring harness, there should be enough wire there.
Mine is tight as I used an existing hole in the firewall to mount it - I can always lower it if need be.
Ideally the coil should be up front next to the horn on the passenger inner fender, that way it will have a stream of air coming from the fan.