UPDATE!!!
Been dealing with fires at work and working extra hours for the last three weeks so I haven't had much time to do anything. However, I made a go at modifying the reinforcement ring with what I had...
As one can observe, the C5 transmission pan is not fully identical to the C4, and it has one serious hog of a corner, which has to be accommodated. On the ring, I marked where to grind, and it took about twenty minutes of grinding with the dremel, bench grinder, and finishing with a file to get the ring to fit the pan- and what do you know, it fits!!
There isn't a lot of room to play with, a few of the bolt holes are definitely not lined up the same.
1. Should I widen the bolt holes?
2. After grinding, some of the metal stuck to the pan side of the ring, basically welding to the surface. They're small bits and I can probably get them off with a little elbow grease, but could a little bit of this hinder the ring's function?
3. Also, I may need longer bolts, will this be a problem if I do?
I'd love for this to work.