Krumrie was well-known for his incredible hand strength, which should be an essential part of training programs for the game of football. Krumrie’s specific grip routine can be found in The New Bodybuilding for Old-School Results by Ellington Darden.
Speaking from experience, a neck helmet trains the head and neck muscles in a unique manner and is an excellent choice though it is not without its disadvantages. To build the strongest possible neck a variety of equipment and techniques can and should be used, including plate-loaded neck machines, manual resistance, neck straps, jaw and teeth lifting, isometrics, and head stands (this list is by no means exhaustive). Keep in mind that building the strength and size of the neck is like developing any other muscle group, incorporate the overload principle, train progressively and recover properly and your collar size will inevitably increase.