Les - Big Brae would make sense - I think a brae and an upland are broadly the same thing, although I think brae tends to be "leathad". It all dates back to a time when there weren't many man made features on the land, so people had lots of names for subtley different types of landscape.
Big eighth farm would surprise me - chiefly because it doesn't make much sense in English! Although the Gaelic for eight is "ochd" - pronounced "ochk", I've never heard of a form that means "eighth farm" - and it would be very unusual if it were a Latin-Gaelic hybrid!
Place names are often difficult to translate, though, because they have been re-spelt and pronunciations change over the years. A few are beyond doubt (Ardbeg, for example) but in most cases it is an inexact science. For example, Glenmorangie claims to be Glen of Tranquility, when I can't see any link to tranquility. I could see Very Windy Glen, though. And Dalwhinnie claims to be the Field of Meetings, or some such. I think it is probably the same as Dailuaine - The Green Field - which would be pronounced Dal oo ah nya in Gaelic (you can then see the similarity to Dalwhinnie).
Sorry to go on - I find it an endlessly fascinating subject, not least because it sheds light on what ancient Gaels felt noteworthy and important.