Blown head gasket? What is the casting number of the head? If it's 603 010 14 20, have the head thoroughly checked for cracks especially in the areas around the #4 and #5 valve seats. If the leaking fitting suggests the engine overheated, consider replacing the head to avoid more work in the future. If the digits in the -14- position are higher than -14- there's a good chance it survived overheating. If you found coolant in #4 or #5, toss the head.
The heater core line/fuel thermostat attachment to the head goes into a fitting (601 203 01 36 with washer N 007603 022100) threaded into the head. It should be a simple matter to replace the fitting. Or are you saying the attachment of the fitting to the head is compromised?
AFAIK the tensioner rail pin doesn't extend to the front cover. Maybe yours is different from the ones I've seen. Did you drop or rest the head on the tensioner rail while removing the head?
Sixto
93 300SD 3.0 - comatose