I've done three MGB diffs, all Salisbury. In each case, there was only one seal available, although this was years ago. Probably best to get the old one out for examination first.
As Arwyn said, a collapsible spacer sets the pinion position, and if you over tighten beyond that point, it should either be replaced, or I do know one case where it was removed and de-collapsed, then reused. But this is hassle.
The ones I did, I marked the nut and flange, counted the turns off. Changed the oil seal. Counted the turns back and tightened until the marks aligned.
The second one was little more than finger tight to be honest , just nipped up from that. A bit of Loctite to make sure, but it was fine.
