Yep the AU # is for your heads. 68 Merc 390 4v/high compression 2v. Measure your cly bore to verify as the C6ME-A casting number is used on the 325/360/390 and 428 blocks. 4.00 is 352,4.05 is 390and 360 and 4.13 is 428. These are std bore sizes so take overbore into account. This way there is no question what block you have. G.