I do like the 1976 and earlier 350 blocks . A lot of the early blocks had high tin and nickel in the cast-iron . Also called 010 / 020 blocks . The tin was used to help the metal flow into the casting mold . And the nickel was used to make the cylinder barrels harder and stronger . If you lived near me I would set you up with an older 350 block . But if you have a good late 5.7 block may-be a 4 bolt main ? And you just can not find an early 350 block . Use the 5.7 block with the seal conversion . If installed correctly it should seal just as well as the early blocks do . And one good thing with the 5.7 block . Most of them were set up for OEM roller cams . This may-be the camshaft type that you will want to run on this build . One more thing with the 5.7 block . Most every one of them are not drilled for the fuel pump shaft .