not trying to argue with the points you made, just saying that the heads listed are NOT needed to make the power hes asking about. plenty of guys out there running 9:1 400's making that much poer & more... its more about the combo of parts than just the head number you pick.
most 6x4 heads come in around 89-93cc, shaving just .030 off them gets them at 83-87cc, plug those numbers into a calculator & you come up with right at or even above 9:1. deck the block or shave more or get better pistons & you are easily above 9:1, closer to 9.5:1. with the right cam & common parts that is easily at 400+hp & if tuned right can easily manage 91 octane. plus most 400s today are at .030 & .060 over, helping the compression even more.
so the 6x heads are still one of the best value/performing heads out there, mild port work & gasket matching doesnt cost that much, most machine shops charge around $75-$80/hr & could do mild port work for well under $500, plus a good performance valve job for under $500.... for the price of some of those rare heads you can have a set of heads done to easily reach his power goals.
shoot i have a basically stock built 400 (406) with 6x4 heads & a very mild hyd cam, block is decked only .005 for clean up & heavy trw forged pistons in my 78 ta that makes almost 400hp, runs low 13's all day long at 105mph. its not "fast" by any means but is stock driveline & heavy, with just a little more work it could easily be at 400hp & if built with the intention of reaching 425-450 noyt much more is needed. this is with a stock iron intake & a q-jet!
bottom line is the 6x heads can & will support his power for far cheaper than the heads listed & much easier to come by. pick the right parts & you can get to a 400+ hp level without much thought, work or money. the 6x head is one of SD performances favorite iron head to port & make 500+hp, & they are one of, if not the best pontiac head porting companies out there.
play around with some numbers on a good calculator like this one & you will see 9-9.5:1 is not hard to reach at all, & from there making the power you want is not very difficult.. this isnt rocket science & does not take expensive rare heads or parts to do it.
http://www.wallaceracing.com/cr_test2.php i did a quick check & using a .030 400 (4.15 bore) 85cc head, which is a 6x4 shaved .030, common felpro .039 head gasket, .005 deck height & came up with 9.15:1 compression. just my opinion of a provem way to get the comp needed for pump gas 400+ power. skies the limit on the other parts you chose.