It's common on older bikes to have narrower spacing between the rear dropouts than newer bikes. This is a common problem when you need to replace a rear wheel on certain older models. Does the rear wheel look newer or non period correct when compared to the rest of the bike? If so this is likely your problem. What you should do is get a helper and one of you work on getting the wheel in the dropouts while the other pushes the rear seat and or chainstays outward. Usually it doesn't take too much and it won't hurt the frame any (especially if its steel, if its aluminum i wouldnt push too hard). Good luck