If your new sender is too big, and you don't want to drill and tap; that's what you're looking at. You could jury rig a bunch of plumbing fittings and adapters to get you to the larger size, but then the sensor is so far removed from the coolant flow that you won't get an accurate temp reading anyways, negating the reason for having the gauge in the first place- plus you run the risk of all the extra fittings keeping your sensor air locked and not getting a reading at all.
