Help with Short shift kit for 996

[ScottKelly911]

My friend just bought an '01 996 and wants to put in the B & M Short Shift kit. We've read that there is an "In car" kit and an "Under car" kit. Can someone please tell me what the difference between the two is. We're looking at the differences between the usage/feel, what the cost diff is, how different they are to install and to make sure he gets the correct part number depending on which one he gets. He ordered part number 45135 from Gilleto Speed Shop in South San Francisco but it's not too late to change if he changes his mind. This is one of my best friends I've known since 7th grade and it's great to see how excited he is about his new P-car. Even though his first exposure to the marque is a water pumper, he's as excited about his car and being in the Porsche family as any of us would be about being given a '73 RS, it's truly refreshing and is a nice reminder that we love these cars. Thanks for your help.