I just had to go through this year my car is tuned so I removed the tune and had to unplug the battery for 4 hours so the KAM can be reset then I ran it for 80 miles until all the emissions systems were ready for smog and it passed fine.
Unfortunately the housing from the GT cannot be used because of the way the water outlet is positioned it would hit the alternator and yeah the "oil cooler" is actually a heater there was a guy that had the GT housing modified to have the outlet extend around the alternator.
Heres the correct part number for the gasket for reference
FEL-PRO 70801
I rebuilt the oil cooler since I was there already it was pretty simple I wanted to get rid of the cooler but you would have to do a remote oil filter which I'm not a fan of unless you had someone modify the tube on the...
for sure its the gasket between the block and the housing its a common failure on ford vehicles of this age I've replaced two of them one in the cobra and the one on my expedition in the past 3 months .
For when the heater core leaks inside the cab they put an outlet to leak out the coolant so it would not be pouring inside the car and its right next to the bellhousing. Its the same outlet that the AC condensation drips out off