Which file should I edit to insert a text + ad in single post?

[Coume]

Hello,

I have many posts indexed by Google as single post i.e.: http://www.mythtvtalk.com/forum/9238-post.html

Which file should I edit to modify the first row to make it more obvious that it is part of a larger thread to invite the user to visit the main thread?
And which file should I edit to insert an ad in this one post page? I want to insert the ad as I do in my first post when viewing the classical thread view... ( http://www.mythtvtalk.com/forum/general/2414-mythweb-19-database-error.html#post9238 )

Thanks in advance for your help.
Ludo