[burnin oil]

The balance will only need adressed if the flywheel/flexplate is mismatched. All 400s were automatics so a flexplate with the factory wieght welded on is fine. Now manuals did not exist factory so there is no cheap flywheel. This brings the balance issue into play. The 400s have a unique balancer and flywheel unless it is converted to internal balance. This means verify what it has. Nothing wrong with the stock heads if they can be reworked cheap. My first 400 build had 882 casting heads and was a real runner and got 12-14 mpg. I will caution you on those 23cc pistons and 76cc heads. Your compression will suck bad. My motor was flat top pistons with the 4 eyebrow reliefs and 76cc heads. With a good street cam it ran on 87 octane fuel. It could have used a little more octane though. No detonation but cheap fuel would cause a little dieseling. This combo is a little high on static compression. Being the pistons are being swapped the whole rotating assembly needs rebalanced. Back to the heads. You dont mention budget but it sounds like this is a budget build but heads are where the power is. If getting the heads reworked is going to cost more than $700 bucks you may want to look at better aftermarkets. I only say this because this is almost the starting point of things like iron eagle. By $1100 you hit jegs heads which are actually on par with AFRs. I have done many rebuilds with stockers in the past but would not do it any more. The price is just so close any more that i can justify stretching the budget there. New ishome just lighrt years ahead of the old castings. Decent heads with some hyperytetic pistons are a good street motor recipe.