golden- I think my 46 in just about like your 40 so I may be able to help. It's been a long time since I have looked at my door latches but if I remember right it isn't that hard to install. There is a left and a right spring so you have to have them on the correct side.
Tell what I'll do, rather than try to explain from memory I'll pull my latches out of the box and take a couple of pictures to email you. Give me a day or two and I'll send something out to you.
George