The GTX 260 has less vRAM (896MB vs 1024MB), less stream processors (192 vs 240), smaller memory bus (448-bit vs 512-bit), less TMUs (64 vs 80), and less ROPs (28 vs 32) compared to the GTX 280. I think it's pretty obvious that the chance of success here is less than 1% (I never say never though).
Oh yeah I forgot, the GTX 280 probably uses faster rated memory with different timings compared to the GTX 260... after reading all the inherent hardware differences you still plan on going through with this, then I wish you the best of luck.