you need to find a buddy that has done this before & get him to help & show you how. if you didn't know this simple little detail, you might end up in trouble real fast. not being mean, just giving advise.
also, you don't put the pistons in with the block upside down. put the left bank level & do 1-3-5-7, tightening the rod bolts to about 15-20fp as you go. then level the other bank & get 2-4-6-8. after all rods are on the crank, flip the block upside down & then torque the rod bolts down to the correct spec. this is just how i do it, this will get you started in the right direction.
one more thing, make sure you have the upper half of the rear main seal in the block (if it's a two piece rear main seal block) before you drop the crank in & torque it down. a bunch of people forget to do this for some reason on their first build.