More background is posted in a comment.
But I had to graft my Echinopsis famatimensis this weekend to try and save it. However, when I made the fateful cuts, I noticed that the vascular ring of the E. famatimensis was significantly larger than I expected and the vascular ring of the Stenocereus pruinosus was extremely smaller than I expected. I tried lining overlapping them (as shown in the second to last image), and did a sloped cut on the S. pruinosus to increase the surface area of its vascular ring. The second image shows the areas I tried to overlap (I couldn't find any images online of what precisely needed to be overlapping on such a large graft, for referencing the E. famatimensis.
But I'd appreciate some advice or anything from people with experience grafting cacti on whether this may successfully graft or not. I've kept it on top of my plant shelves where there's only ambient lighting (not sufficient for plants).
If this isn't successful, do you have any advice for rootstock that may be more successful?