4.5 was the standard rim width in those days. Only heavy-duty packages like Formula S got 5.5. Dart GT was a trim level, not a handling package, so 4.5 was probably stock.
Note that any reasonable size of 14" tire is going to bulge out of the spare tire well, anyway, the rim width doesn't matter much.
10 years ago I purchased my 68 Dart GT and I notice that the spare rim is only 4 1/2" wide. I thought that the spare rim is 5 1/2" wide. Is it possible that someone put a wrong rim into the Dart?