Id say you have a seal/ cowl rust problem . If you had firewall issues you would know it

The firewall should not let air come in unless it is rotted through under the hood (where it can be seen) .
Id see if some local Mustang alumni might be able to help you out .
Not long ago there was a guy on this forum that was getting fumes and hot air coming in the car and by posting here he figured out his cowl to hood seal was missing . Replaced it and the problem was fixed .
You could simply have a bad seal around your steering columb , hardly need a complete firewall to fix that .
The Firewall about requires removal of the cowl to get to it . Torque boxes would need to be in top shape or replaced before cutting , unless you have a frame jig to hold it all in place .
It's not a easy job but Im doing it .
Will I do it again ? nope
