I think you would be wasting your money to mill the heads to try and gain compression when using stock pistons. The pistons are so far in the hole that the gains will probably not be worth the money spent. Those 400's are good engines...but they are advertised as 8.2:1 compression, but if they are like every BB Mopar I have dealt with, the compression is likely actually much less. I did some quick calcs and assuming the piston is .120 in the hole like a smogger 440 usually is, I get 7.58:1 compression with a .020 shim gasket. I get 8.17:1 compression by milling a typical 906
90cc chamber down to 80cc's.

If I'm wrong on this....someone please correct me though. If your set on using the stock pistons, that is ok...I would just use a stock shim style head gasket and not bother milling for compression. To mill enough to matter you would have to mill the intake side as well and this will all cost money. You could just spend a little more and buy some cheap ($350ish) KB hyper pistons like Greg suggested and get much more gain. But you will have to rebalance because KB's are lighter than stock pistons are.

Or just leave it all stock and live with the compression and use the smaller summit cam listed. It will still have good/ok power.