How much power is a stock setup good for?

I have a completely stock 68 barracuda and just wanted to know how much power I could put into it before I have to start looking for suspension, steering, and chassis upgrades? What would need to be done first? I'm just starting my build and am just looking for a little more info on how durable the car is. I would like to keep everything stock until I put an engine in (goal is around 400 hp) but don't want to hammer it down and tweak the body or blow out the rear end, you know what I mean?