I haven't had a 56 for many years but for some reason I'm thinking the cap is vented. There may be a difference among the 3 years but I'm almost positive my 57 is vented. Can't check because I'm away from home, but a search in the supplier catalogs may state the cap as originally being vented or non-vented.
All the rubber hoses and all fittings should be checked, and the road draft or down draft tube is a source for odors as well. Unless the car is shown I would recommend a PCV conversion to eliminate a source of odors and drips. This should be easy enough to track down.