I wouldn't spend any serious money on a 557 block. Any other 400 block that's in good rebuildable condition is OK.
No reason yet to believe it is a 557 block based on known codes.
If a relatively stock rebuild is a 557 block an issue? OP has provided little to no input on goals for the engine other than as a replacement for original engine.
1968 400 Coupe, verdoro green, black vinyl top 1968 400 Convertible, verdoro green, black top 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver)