So, you could run 5 and 7 together with new wires and when it starts to cross-fire, it's the indicator for needing new ones
Let me put it so you can understand as it seems you don't quite have a grasp of what I was trying to convey. IMO Running 5 and 7 together is a non issue with todays wires due to the superior materials they are being made with as opposed to the old packard wire that came stock.
Not only do my 5 and 7 run next to each other, they also cross at the top of the distributor cap. So, again in my opinion, if you have a new wire that's cross firing, you either damaged the wire, or you did one crappy job of making your new wires. Or you bought crap wires, because quality wires don't leak.
Rocky