Well, after giving up on the tail post for the night and getting a good night's sleep, I went back the next day and re-measured everything. It turns out that through a combination of measuring errors and angles, the tail post position was too long after all! This meant I could fit the post as per plans and make it look nice as well. I must have been really tired when I was trying to figure out the initial cuts and position and made some avoidable misteakes...oops..I mean mistakes.
I've found on this project that measuring and building accurately really pays off through the process. When something I'm doing just doesn't make sense, I step back and wait awhile. I know the basics are covered, so if something is out of whack, it makes itself known by inaccuracies right away. In this case, the tail post position in relation to the rest of the fuselage was just not right, but I just needed to figure out why....and I did.
Now, on to the tack welds and getting busy on the rest of the fuselage...