disregard that last post i just realized you were talking about the block casting, sorry about that
do you have the motor in complete form, or is it just the block, what all do you have. the stock crank and rods can easily handle 400hp, you could go with a cast or hyper piston in my opinion.
781 bbc heads are good performing heads and really wont need any work in order to get them to handle 400hp, you may want to get them checked out by a machine shop to see if they are in good working order but other than that they should do fine stock. i will recomend a hyd roller cam, one with around 214-222 duration @ .050, advertised duration around 268-274, you should be able to use about 9.0:1 comrpession and stock stall, i would upgrade the rear gears thought to around 3.42's if they aren't about that already. you could go with a performer intake or if you can fit it you could also use a performer rpm intake and about a 750cfm double pumper.
this combo should give you about 390-415hp and 515-530tq (if not just a bit more, as i think my estimation may be a little modest, but not by much)