Use the bigger one and try to flip the support around 180 degrees. If you already tried that or it doesn't work, just use some washers to space the rear support. When I had my 1515/1y in my revo, I didn't even use the rear support....
I won't fit, there is not enough material on the rear brace to grind enough away or it would be paper thin ...so I don't need the rear support ? guess the finned cans on 1515 neu's are just real big...
I got one of the first mounts Mike made for the revo and it only came with one support that was designed for 36mm dia. can motors. So, it did not fit and I really did not need since the motor rested against the bottom of the mount...