Mike, I really want to help, not that I'm an expert or anything and Chryco certainly knows more than I'll ever know but I spent a month and 3 steering gear boxes recently getting mine straight, so if I can, I'll tell you anything that I learned.
My coupling must have been modified (it's on so I can't check now) because I could slip it on to the worm shaft in any position I wanted to. You already know that your splines are still good though right?
On mine, someone before me had installed the coupling on the steering column upside down. When you placed my steering wheel straight, the index on the coupling was upside down so the person installing made the mistake of turning the worm shaft 1/2 turn to match the straight steering wheel then put everything else together. That resulted in 1/2 turn being taken away from a right turn and added to a left turn. Then they adjusted the tie rod ends to put the wheels straight.
How I fixed it was to flip the coupling over (housing only since it's symetrical) then I made sure that the index on the worm shaft was at 12:00 and slipped the coupling on. With mine I also had to make sure the coulmn was straight because it would go on in any position. (This was probably true because the previous install ran into the problem and couldn't figure it out so he filed the index off the coupling.) But even with the index, the coupling might be on the column upside down and with the wheel straight would put the index at 6:00 which would make your install 1/2 turn off.
One way to kind of double check is to look at your tie rod ends too. I would think that what you describe would require you to have one side almost all the way in and the other side all the way out.
Oh, I forgot (I doubt that this is your issue). Did you count the revolutions of the worm shaft before you put the column on? The gear rotates 3.5 turns so the middle is 1.75 turns from either stop. If it was a full revolution off then you would hardly have ANY turn in one direction, that's why I doubt that you have this problem.
I don't know if this helps but I'll keep reading, and for now I'll keep my fingers from typing any more unless I read a specific question
Good Luck Mike.