G'day guys,
I just thought I would report my findings for anyone who is planning on using a skyline stumpy gearbox on a CA20 bellhousing. After putting the bellhousing over the gearbox it turns out that the smaller bearing visible in the bellhousing (The secondary shaft main bearing) is actually smaller than the cast hole/support in the bellhousing. This effectively means that the secondary shaft would only be supported at one end if something wasn't done about it!
Now all you have to do is get a prefferably new bearing from that same shaft from the CA20 gearbox. The NGK number of the bearing is B22-19C3UR. To swap the bearings over it is best to have someone press the old one off and the new one on but it involves the whole box to be pulled apart and can be costly.
People are going to cringe but I managed to get mine off reasonably easy with some wedges and hammers. The new bearing taps on quite easily.
Now the moral of this story is, use the S13 box from the start. I wish I did
It's a stronger, smoother box and even though it costs a little more in the beginning it would be marginal once you have considered the time and extra money involved in modifying the skyline box.
I hope this is helpfull some somebody!