I went through the same process recently and was told buy my local KTM/Berg dealer that most KTM Akrapovics will fit, basically stick to 04-07 KTM's EXC and it should be right, as a double check get the measurement where the header pipe goes into the Akra
The other thing to remember is the seam will be shown and sticker will be on the tyre side when fitted to the Berg
I did notice some Akra's for some KTM's have a dent built into them to clear something on the bike so look for this also
hope this helps