It may be possible that you rolled the lip on the seal going over that splined end on the differential shaft. It is very easy to do... I rolled one on my 310B even though it is slightly different,and used a slick trick I learned from another site, to not damage the seal. Yup, it happened anyway, and I was careful.
If you care to try it again, wrap the splined end of the differential shaft with electrical tape. Start from the inside, just where the shaft is smooth, and wrap outwards. Then apply some gear oil to the surface of the tape.This will give it something to slide over, and no edges to catch on. The seal should slide in past, where the tape ends. Leave a tail on the outside end, and when things are in place, you can pull that tail piece, and it will pull right out of there.
From the looks of the exploded view @ Case/IH, it looks like the seal is located in the inside brake ring. Am I correct..??
I got mine right the second time on the 310B, and within 6 months, it was leaking again. I do believe this is the nature of the beast, after talking to some Case collectors.