please don't modify the power meter, you will break it. The inner diameter on the Classic is not by chance - it's the biggest that was possible for that generation.
You will still be able to fit it on the vast majority of frames, BB386EVO is unfortunately the most complicated to handle from a power meter interface perspective. BSA, BB30, PF30, BBRight and ITA are all fully compatible, BB86 in the majority of cases.
For cases like yours I recommend the FSA BB386EVO models we brought out at Eurobike - they are the only ones that are fully compatible with BB386EVO frames, as the power meter plane is outside the bottom bracket area.
official power meter of Movistar Team