You will need to have more than just a mating flange between the flex and the Escort header. The Escort header exit points in a different direction than the flex entry so you need an extension or bend to get the header to match up with the flex.
If you could see them from the side the header flange is perpendicular to the ground while the flex sits 6 inches (guess) away from it at an angle facing towards the bottom front of the car at about a 45* angle (another guess).
There is plenty of pipe between the muffler and flex to put a converter in-line after the flex. This should be do-able provided the heat doesn't bother the underside there and you extend your downstream sensor wires....oh, and you'll need a sensor bung welded after the converter too.
To install an Escort header you only need a custom piece of pipe to go from the end of the header to the flex. A muffler place will be able to make this for you and they will have the flanges they need.
To add a converter after the flex, find the narrowest, shortest converter you can, take it to a muffler shop and tell them you need it where that long pipe is, with a sensor bung after it. Again, they will have the flanges they need. After that, just get a longer sensor harness and viola!
You will still have a restriction from your OEM flex pipe though. To get a higher flowing flex it is another custom deal.
Panties aren't the best thing in the world but they are next to it.
I would rather drive the slow car Rusty built than the slow car I built.
Do your own repairs, it's not rocket surgery !!