Too late to edit the last post, so I will try to clarify by saying that there are two size speedo gear housings that fit into two different tailshaft (extension) housings. Because the tailshaft housings will interchange, either tranny may now have either housing on it.
I don't think the gears pitch will allow a small housing driven gear to work w/a large drive gear or vise versa, if it did the speedo error would be huge.
The speedo gears will be in sets that have considerably different counts to them. You should be able to select the right size of gears by the driven gear housing size you have.
I hope this doesn't confuse the issue worse.