just rebuilt, I will assume this means the block is dry. Put oil in it, remove the distributer, using an oil priming tool I would put pressure on the system just to pump up the lifters. Now the fun part. Just a recap after the system has had pressure on it I would look at the rockers to make sure the oil has made it's way to the top of the engine filling the lifters. take a cylinder anyone you wish to start with. Start with both valves closed , rotate the crank until the exhaust valve JUST starts to open. At this time go to the intake on the same cylinder and adjust that lifter. Back off the nut until you can feel play in the rocker. Count to 5 or 10 just to make sure the spring in the lifter is expanded, recheck to make sure the rocker is free (loose), Now slowly bring the nut down until all the play is gone (0 lash ) from the rocker. Now continue 1/2 more turn and the adjustment is done for the intake. Now rotate the engine again and notice when the now adjusted intake begins to open. Stop there and now adjust the exhaust valve as described above. Then continue on to the next.
Two foot notes:
1 GM spec. is 3/4 after 0 lash
2. a rule of thumb is that if a valve just begins to open the opposite will be at 0 degrees or fully close, on the flattest spot of the cam.
Hope I have helped you out, if anything is unclear jut PM me with any questions and I will be glad to give you more information or clarity