I'm in the process of doing the same conversion as you. I found that when mounting the trans cross member there will always be two holes - one on each side that line up between the 85 frame and the diesel cross member. I used a magnetic base drill to drill the new holes in the frame.
Instead of reversing the skid plate mount I found it was easier to elongate the transmission mounting holes. I used a milling machine but a grinder or a torch cut and filing was easier for me than slicing and reversing the skid plate mounting tab.
The next thing you will probably encounter is the drive shaft pillow block bearing mount from the 318 driveline. This is normally a plate that mounts to the cross member. In order to maintain the proper drive shaft/driveline angle I had to mill out the 1 inch rise and then weld the plate back together with a slight downward tilt toward the rear. Then to fine tune the angle you shim the front two mounting bolts to elevate the plate upward toward the front. I used an 89 Dodge Cummins 2WD 2 piece driveshaft. I had the front shortened and the rear section retubed. The drive shaft shop told me they used the same tubing thickness that they were using on a triaxle dump truck driveshaft.
I also made a template from the donor trucks firewall to locate the new holes required for the newer wiring harness and the clutch master cylinder mounting locations. There is one hole that you have to fill and recut because the outer portion of the holes overlap.
The template for the clutch mounting bracket inside the cab is also a good idea if you have to drill the new holes.
Also, historically Dodge lost its major contract customer for their crew cabs in 1985. That customer was the US military and GSA services. GM won the contract and this is why there are so many GM crew cabs starting in 1986.
Thanks for the heads up! I didn't need to shorten or lengthen the front driveshaft as it sits in the exact same spot with the 4wd conversion. I would have elongated the holes on the mount plate as that is easy, but I run out of plate so it is not possible. How far are you in your project?