Glad you got it in! Is this a "Magnum" related interference, or possibly a header brand/shape deal? The magnum blocks are hand picked to have the LA motor mounts. (Some do also have the magnum bosses, bit those are pretty flat to my memory).
I have always laid my headers in place before dropping the engine in. Was that possible? Just asking for future installers. Most inexpensive headers I have seen, have the drag link going through the tubes. Of course never mentioned in the instructions...and takes either knowing it already, or doing it the wrong way to realize it.