If you plan on making anywhere near 400 hp you better put some money aside for your drivetrain too. A stock 8" wont take that kind of abuse, and neither will a stock c4 (if thats what you have). And having 400 hp worth of engine is a really bad thing if you only have 170hp worth of drum brakes.
If you don't wanna have to rebuild your trans and rear, keep it under 300 hp, which is a good number for a 302. A nice heads, cam, intake package should put you there.

 

Great answers... now I know what to tell my son when he wants to do the same to his 68's 289. He just bought a 600 cfm and is planning a new cam of some sorts. From the sounds of it, any rust that is holding the frame/body together doesn't stand a chance without upgrading for strength.