Thanks Jim, Harlan.
I got out to the hangar again this afternoon, and a friend from a couple of hangars down came in to look at them for me too. At first glance he thought they would probably be fine, but after a few minutes more chatting and looking we came to the realization that I had overtightened them. I wasn't aware of the tightening procedure for NPT fittings.
We pulled out the two fittings I had put in, and inspected the threads on the female side and they were a bit off from the overtightening. Nothing that couldn't be fixed by re-tapping I was told, so thankful to have heard that.
I've just ordered a tap, and replacement fittings too, and they should be here by early next week. I'll re-tap all of the holes in the header tank (not just the ones I put the fittings in).
I'm going to shift focus now to installing the auto-pilot servo kits while waiting for everything to arrive.
While I am stalled on continuing to work on the header tank, is there a trick to getting the elbow fittings to point the desired direction when installing them, or is it a trial and error type of thing?