Here's a shot I took before the original conversion (note this is the K20D), you can see the three Allan bolts through the cutouts in the main board. The one just to the right of the viewfinder is partly obscured by a flip-type ribbon cable. You can get to this point at very low risk if you follow the service manual already posted. I use a sheet of paper for making notes as to which screws are which, and place the screws and notations on a wide strip of masking tape so they don't roll away. To get the back off you'll need to remove quite a few screws and note their positions since there are several different sizes. The top cover can be lifted slightly as in the picture, and the bottom cover can be removed with 5 small screws and 8 slightly larger ones.
To remove the back there is a ribbon cable connector underneath, then the back can be gently pulled away from the body - you'll find that you may need to ease off the front panel as well to free the back.
I'd say that the bolts towards the bottom of the body will jointly take care of horizontal adjustment, but exactly how much adjustment is needed and in which direction is unknown. You can leave the top one, since the cable that's in the way is quite a fiddly one. Bear in mind that the image on the sensor is actually upside down, so it may be possible to infer from your sample shots whether the sensor needs to be moved towards or away from the lens and which bolt is resonsible. Or you may ruin it. Note also that the bolts are surrounded by some glue so they don't move. A 1.5mm Allan key turns them fine.
I accept no responsibility for your camera, etc etc. I am simply providing some information to allow you to assess the problem. Please don't embark on this unless you really understand the risks of messing up the camera. Ask me more questions if you aren't sure.