If it were ME, I'd ditch the oil cooler all together....unless you do any road racing of something but honestly, when was the last time you heard of a motor failure in a 5.0 b/c the "oil got too hot." The benefits....you can ditch the lines and everything ugly and NOW you can run a BIGGER GT style filter.
the fl1a filter is for any old 5.0
the 5.0 cobra oil filter is the fl-300
Just pull off the filter, unbolt the oil "cooler" block from the bolt. NOW, a standard FL1A Motorcraft filter will bolt RIGHT on! As for the line, remove them all together and get a small "L" shaped heater hose to go from the T-Stat housing to the rigid heater core lines (JUST LIKE on a GT). Its that simple.
Now, IF you need to seal if up and you insist on keeping it, I'd try the O-Ring off an old oil filter bolted down between the cooler block and engine block.
