Newbie Needs Info Help Please

Hello, i have a 67 barracuda 273 with basic bolt ons, 727, and original rear end. She will almost be a daily driver with ocasional trips to the local drags. I know for sure i will want disc brakes up front and i know it will change the bolt pattern.
My question is on the rear end. how can i find out what rear end came stock on my car, and when i go to the big bolt pattern, what donor cars should i look for to get my rear end, what size rear end and what gear ratio do you suggest. what modifications will need to be made for this to work. any information is greatly appreciated. thank you in advance.......Karlo