The 670 and 680 are already pretty much the same card, just the 680 has some more CUDA cores. If you clock a 670 to 680 speeds, you will get 680 performance in games.
760 to 780, nope. Entirely different GPU's, memory bus and VRAM in them. For flashing to work, the hardware needs to be identical. Thats why 6950's could be flashed into 6970's, they were the same thing except some shader cores were disabled. AMD then later started cutting the shader cores off the 6950's, then suddenly people couldn't flash the cards anymore.