I am confused by your initial post - is your main objective to replace the steering coupling or the gear box?
If its just the coupling and you got the upper allen head bolt off then try this for the lower bolt: the correct angle is the key. using your allen head bit with extensions get to it from the top by the firewall. Have someone give slight turn on the steering wheel till you get the straightest angle to push the allen head bit in the bolt. You will not be able to do it entering from near the brake booster, it will have to be almost a straight down angle from the center. Once you've seated the allen bit as best you can, give the extensions a few whacks with a hammer to fully seat the bit all the way in. Now try turning the bolt out.
These allen head bolts fill up with dirt so that you sometimes can not seat the drive bit all the way in. The impression you get is that the bolt is stripped. These bolts are not that easy to strip, there are good sharp edges at the bottom of the bolt openning. Also, check the allen bit you are using - if it is old and worn chuck it and get a good one for $12 at Sears. That will make the difference. Good luck. If you get the bolt out you should be able to remove the coupler by just removing the 3 bolts that hold the steering gear box in place. The gear box should move enough to let you slide the coupler off and insert a new one.
If on the other hand your objective was to replace the gear box and you removed the upper coupler bolt then you should have no problem sliding the gearbox with coupler off the steering shaft spline. If you find it difficult to manuever then just loosen the steering wheel shaft, pull it up a few inches and it will come out of the coupling. I simplified the description, several things to be careful of like lining up the steering shaft so that your turn signals engage/shut off properly, etc...