I think you are making the right choice as well. But I don't think you can bore it to a 400. The bore of the 400 4.12 and the 326 is 3.72 with both having the same stroke.Thats about 4 hundredths over while most people stick with 30 thousandths. Since the motor may not have that many aftermarket parts, then try to find out it they 326 and the 327 have interchangeable parts. The 327 is a hell of a motor and if you could combined some of the aftermarket parts for that you may have it made. They can easily get 400+ hp. Oh yea what type of heads does that olds 455 have?