Group 42 was the smaller, lighter battery used in most of these cars.
Group 41 was used in diesels, cold weather area cars and lots of A/C cars.

Beware that because of the price of lead, many battery manufacturers are "consolidating" sizes and you may be sold a group 58R battery that is very similar in size to a group 42 but lacks the part of the battery tray case that slides under the backside of the battery to hold it in place.
YMMV
Good luck.
cheers
dave parker