Have you measured it? I don't remember 428's being used for "irrigation".... there were many shortages of the castings and even Shelby had to install 390's because he couldn't get the 428 blocks.. In any event, you need to decide the goal(s) and the budget! An aftermarket block has some advantages for sure!! Same as aftermarket heads/ intake/cam etc. but can your budget support that?

If this isn't part of a "numbers matching" type restoration, and the car you're assembling doesn't require it, I'd really consider the new block/heads!

Also, at that website I linked for you, Barry R. is there constantly, discuss your plan with him as he really is the FE Guru!