Bob-
I had a similar crack on my '69T mag case 901. Is your case Al or magnesium? looks like mag to me from the pic.
I replaced my housing with a 914 housing. The swap required relocating the vent and swapping internals/nose cone. I had everything out and apart already and needed the 914 for some pieces to do the simplified differential fix so it didn't cost much extra. The most difficult part was getting the pinion depth correct without the elusive pinion depth setting tool.
After I did all that I saw the thread below with a post by Grady Clay. Apparently these cracks are fairly common. If Grady is living with them on his 914/6 race car I wouldn't worry if I was you.
http://forums.pelicanparts.com/porsche-911-technical-forum/358380-911-901-trans-simplified-diff-other-questions.html
You may want to do some searching on the simplified differential issue/fix to see if it applies to your transmission.
Good Luck,
Jim