It is also possible the oil supply tube was not reinstalled during rebuild. A tube runs out of the oil pump and up into the block to supply the oil galley. The oil galley runs the length of the block on the left side about 4 inches up from the oil pan rail. Ther should be a junction block in the front of the engine that sends oil up to the head and the external bypass filter if so equiped.
A 270 gmc will easily take 6 quarts of oil.
