... I'd have to thread the bottom in deeper than it is now to
thread it into the top elbow piece.

You mean, like, thread it WAY down into the one side, so it fits into
the little 8'' space available, then back it out as it threads into the
other side? If so... no. You can't do that.

No PVC either. And I'd be leery of any rubber gaskets either, unless
they are specifically designed and labeled for fuel oil.

Can't you just by a union fitting? That's the first thing I would look
at -- cheap, easy, simple, and probably up to code too, unlike the
other suggestions floating here. But don't listen to me -- go find
someone who knows fuel oil, pipe fittings, and code. Be sure to use the
right kind of goop or tape on the joints, too.