If you check out the Le Mans threads on 10-Tenths they provide a lot of this sort of information - mainly in relation to the 'modern' Le Mans race - but a lot of the 'viewing area' stuff is relevant to the Classic. Also try the following (which I found via the aforementioned threads) - a real mine of information and a credit to those who produce it:
http://www.clubarnage.com/wiki/doku.php
I've attended the last two Classics (and again this year hopefully) but don't bother with Grandstands as I prefer to wander around, so can't offer any first hand advice re grandstands.