Stag makes a great lefty 6.8 (a great righty too). However, there has been much discussion on the M4 feed ramps. Stag maintains that they are not neccessary, and many users have not had problems. On the other hand, many users have and the problems have been solved by the lengthening of the ramps.
The problem is the 6.8 cartridge is fatter than its little nephew the 5.56, and thus sits slightly lower in the magazine (.025" IIRC). Depending on magazine condition or ammo type, the point of the bullet may connect with the receiver/barrel extention incorrectly and cause a failure to feed.
Were it not for the ramps, Stag would be one of the best (for the money) options in a 6.8 upper, and the only left handed option. Of course, if you get one, yours may run just fine and would make a great rifle. If it doesn't, M4 ramps are an easy DIY job or can be done cheaply at most any gunsmith.
To directly answer your question, no, nothing needs to be done to the lower.