To make the 6a box the same as the 6al ..is to buy a $70.00 part..buy you a msd rpm selector module and plug into the box..you can put your rpm rev chips in this selector module...this is the only difference between the 6a and the 6al,,when they built the 6al they just incorporated the rev limiter inside the box itself eliminating the need to buy the selector module......
also the reason i say 600 is possible is beacuse my last 406 made 620..but had a slightly bigger cam and a little port work to heads...so yes i think mid 500's to high 500's possible 600 is there with some little modification..
just remember you start doing to much you cut down on the drivability..
when you get the block to assure its a 400..look on side of the block..it will have 3 freeze plugs..or 2 freeze plugs and a factory boss for the 3rd..if it doesn't have this is most likely not a 400.....
it should make 500 blind folded...where you have an edge is the heads you have ..they will make a little more hp, discipate heat quicker which equals HP and its a little lighter...than cast....
good luck
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "