You just slide the engine cross member forward in the frame.
I had the frame disassembled before blasting / powdercoating when I did mine, so it was very accessible to say the least. If you look closely at the first picture, the frame is upside down. I flipped the engine cross member over on the outside (bottom) of the frame and lined it up with the original bolt holes. It's symmetrical, so it doesn't matter that with it flipped that the left side is on the right and the right on the left. I then scribed both the frame and the cross member and scooted it forward 1 3/4", clamped it and then drilled new holes using the holes in the cross member as a template.
In the second picture, the frame is right-side up, but the top parts of the engine cross member are now clamped in place after doing the same procedure, (cross member now upside down on top of the frame, scribe and move forward). The main portion of the cross member is removed in this picture after clamping in place to make room to drill the new holes.
Hope this helps. It really beats guessing where the holes need to go. Of course, this technique would be a little more difficult with the frame still on the truck...
Edit: I just remembered that there were already two holes in the frame that were pretty close to the new location... but too far off for my liking. I ended up welding up those holes and drilling new ones. You might be able to get away with using the existing forward holes, but I think that the mounts would be tweeked with it bolted up. Maybe someone else can chime in...