You're right I think the rails are much the same, from memory though the rail had to be modified to fit the U12 floor pan.. either that or it was that the bolting locations matched but the floor pan mounting did not. I can't remember who did it though so I can't confirm or deny.
I don't think i have any pictures of it unfortunately, but I mocked something up from memory in 3D. It's a pretty simple mod, you need to make sure though that the bolt locations on your seat can fit within the mounting area allowed by the RHS or flat bar that you're using.
I originally used 50x25mm RHS but it placed the seat too high. I used something like 70x5mm flat bar which allowed me to locate the seat a little further forward / backward as required by using different bolt hole locations on the wider bar.