If it's like our '64 (which it should be) there is a 'pin' mounted to the floor and a 'catch' on the bottom of the water heater. You need to pull out the water heater straight, parallel to the floor without trying to lift it out.
I have some pics on our webpage that may help, check the 'Water Heater'
page and you can see the pin in the floor (especially, pic #4). The bottom of the water heater isn't shown but it's has a 'C' shaped slot that fits around the pin.