Frank, you say the engine is rebuilt,was it disassembled and cleaned properly? A rebuilt engine should not need to be flushed out.Maybe some gasket goo got in the oil hole in the block?The oil comes up a small hole in the deck of the block to a matching hole on the head, then detours to the head bolt which has a reduced shank are like all the headbolts, then detours again to feed the oil tube to the rocker shafts.As Dave says, the oil path through the block was changed several times.
From 38-62, only two headbolt part numbers are listed for 216-261, long and short except for a drilled bolt used when a 59 block is used with earlier parts to reduce the oil flow to the head.
