A 400 has visible buldges in the sides of the block.

 

A 325-350 hp 305 will make you Camaro plenty fast for the street, especially with your drivetrain set up right. Stick with the 305.

 

If your dead set on building your 305 stick with top end parts that are interchangeable with a 350. Intake, heads(possibly), headers, afpr, 1.6rr, forced induction, exc. Personally I would stick with the stock heads and give them a full port (see sittingbulls porting thread) and save yourself the money. Also I wouldnt touch the bottom end. Just my personal opinion.