I've converted a lot of 6.5x55 Mauser cases to 7.5x54 French, and .284 Winchester to 7.5 Swiss. The 6.5 goes from .264 to .308, and the .284 goes from .284 to .308. Always do them in one pass over a n expander, and have never lost a case to split necks. Assuming the .270s are once fired, I would be surprised if they would split with that small a change. If they are getting tired, I would anneal them first.