The booster is the same, just get the disc master cylinder and the correct rod and you are good to go. The 69 style disc master is has an oval shaped lid and uses the long rod. The 70-72 style master has the square cap and uses the short rod. 

I would think that the cylinder might come with both rods   Ask John at Car Quest Clinton or I will for you  

THX guys, I will stop in this week and get it on order....you guys are a wealth of knowledge, and I appreciate your help very much!!!!!!!!!!

Ok so I got a power disc master cyl. this week, and it did not come with, or list a rod for it. It turns out it is the same master cyl. as chev trucks. Any way I removed the old drum one, and looked at the 2, and it appears as though they both have the same measurements for depth, and rod location. I looked through my '72 service book, and couldn't find any reference to changing rods, only minor adjustment procedures. SO I am assuming they use the same rod??????? Will be a bit before I can test it to be certain......