Slant 6 mod's

I would like to hit 13 or 12's. I know know the rear end must be changed out to a larger one and what gears should be put in the rear?, im trying to keep the car as stock as I can, so what in all needs to be done to the engine to achieve this? whats a list of things so I can tell the shop what I want done, trying to keep the cost down as much as possible,

Thank you and God Bless,

MOPAR 4 LIFE