I have a 6, never had a III, my 6 has the split image focus feature, glass masking 6X6 carrier, only complaint is that it is just short of a 11X14. It is my go to enlarger for 35mm and 6X6 while I use my Omega D3 for 6X9 and 4X5, or if I want a full 11X14 from 35 or 6X6. I had my 6 lens board drilled to take my Wallensake 50 and 75mm lens.
Key question in my mind is are both in alinement? As consumer level enlargers not a lot of adjustments are possible. If you have grain focuser, find a negative with somewhat larger grain, a 400 or 3200 speed film, focus in the center then check the 4 corners to see if all corners are in focus. If not then look to see what can done to bring the negative carrier stage in alinement with the base board.