I have a 69 chevelle ss and i can't hook up with 295-50-15. I wanna put a bigger and wider tire on the back to get some traction. How big can I go and what offset for the rim would I require for that tire?  

You want 0 offset and the backset is differant on differant cars, but a general rule of thumb is half the width of the rim is a good backset but this does NOT APPLY to all cars. You have to borrow other people's rims/tires to make absolutely sure. JB