I'm not sure about the Interceptor engines... but in the Windsor family, the 5.0L and 5.8L do not share the same firing order.
(just an FYI on that)
Your ignition distributor cap will be indexed to the distributor housing..... so in essense, you are locked into this if the cap towers are numbered, and if you wish to use their numbering system.
The distributor housing indexing can be placed where you want it to be, as long as the cap towers correspond correctly to each cylinder as per the firing order.
This may mean lifting the distributor, and re-indexing it.
If so.... be sure to re-set the ignition advance.
And since this is an older Mallory unit using the flat cap, be sure to examine the flyweight system.
These are years old by now, and if water or moisture caused any damage, the flyweight return spring values may have become compromised by now.
Rusty and weak return springs can lead to premature progressive advance, and will cause engine damage.
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