"I have a 2 3/4" only Remington 870 and am looking for a rifled slug barrel. All I can seem to find are 2 3/4" / 3" barrels. Can I use one of these or must it be a 2 3/4"barrel only?"

You can use 2 3/4" inch shells in a 3'' chamber. According to Hastings and Remington, you can use 3''shells only in a gun having a magnum receiver.