Brian, yes, good idea. A knife switch large enough to handle the starter current should have contacts with heavy spring metal pressure that will ensure a positive connection every time so I think that would be a much better choice especially for 6V cars.
My experience with batteries in general, even standard lead acid types, is if the car is garaged and used infrequently, and the battery kept charged it will last up to 10 years. I have had my Buick since 1978 and only one battery lasted less than 10 years. I also have a 1965 Mustang in the garage and use an economical Walmart Everstart battery. The first lasted 11 years and the second is now 8 years old. I do keep all my batteries on trickle charge during the winter but leave them in the car.
Steve D