The biggest six volt battery STOCKED locally is a 900 CCA Interstate battery. Should I order something bigger from the truck dealer or is this plenty? 48 months, $39.95 w/trade.
A friend just bought a heavy duty 6v battery that is the right width & depth, but too tall to fit under the floorboards. Check that.
Check this out with your battery box dimensions:
Ive been buying a standard group 2 size battery from Tractor Supply Co. They have stores in a lot of areas. Their logo is the initials TSP on their sign. I think that this size is the correct one for the LaSalle. I use them for the 41 Cadillacs, and the 38-60S. I believe that the height is proper for you, too.
The other thing you might want to do is what lots of the classic and other owners are doing, and thats to use an Optima battery. I just bought one for one of my 41s.