I have a 1968 chevelle with manual brakes disc in front and drums in back. I would like to convert to power brakes. Do I only have to add a booster or is there more items that have to be replaced and what are they???

i believe you will have to replace the rod also and move the rod location on the pedal to the other hole that is on the pedal

It's as simple as getting a power brake booster (these can be had for just over $100 now-a-days from some restoration parts companies and they look just like factory), a new/rebuilt master cylinder or power brakes, yes it's different (~$30 or perhaps cheaper), a brake pushrod (the rod that comes through the firewall). Thats about it ... restoration places would probably have the brake push rod, or someone else can point you in the right direction of those.

Good Luck