If the block was decked on a rebuild then the original numbers are lost. (They could be recovered if the block was etched with acid, but it isn't guaranteed, and the acid destroys that area of the block).
Date casting numbers are found on the rear step of the block in front of the bell housing. They are recycled every ten years so a date code can be for a 60, through an 80's series of blocks. 1984 was the last year a 402 (small bore BBC) was produced. If it is a one piece rear main seal it was cast after 1990 and the displacement was cast into the side of the block in big numbers (7.4 for a 454 block). If it has no provision for a mechanical fuel pump it is a Gen V, it it is a one piece rear main seal block and it does have a provision for a mechanical fuel pump it is a Gen VI block.
1974 was the last year a BBC was available in the Corvette or a full size Chevy car (1973 for everybody else), with the exception of pick-ups. After 1974 all BBC engines were only offered in a light truck, and the heads had a major redesign dropping port volume to increase low RPM torque called a "Peanut Port" head (note light truck oil pan will not fit in a car body and has to be changed, even on a crate motor from GMPP).
Block casting numbers that determine displacement (bore size) were cast next to the date casting in the same area on the back of the block. Two bolt or four bolt is determined by the amount of machine work, as all blocks are the same. Any two bolt block can be made into a four bolt by finding four bolt caps and having additional machine work done to the block (that is why four bolt or two bolt frequently apply to the same casting number, though there are some numbers that were four bolt only such as marine or truck blocks). In addition to having four bolt main caps a four bolt block was machined for an external oil cooler, used on trucks and Corvettes.
All Mark IV blocks are essentially the same from 1967 through 1990 with the different casting numbers reflecting bore size and changes made since 1967 to the block to reflect changes mandated by federal law for emissions requirement. Every change to the casting copes and molds generated another casting number even if it was just to cast a pad to have a hole drilled in the block for a sensor, or to remove casting metal from the bottom of the block to add a longer stroke crank.
All Mark IV blocks look alike alike on the outside; and are different only in casting numbers (1965-'66 had a different oiling system, and rear cam bearing reflecting the difference in how the upper half of the engine was oiled).
Since almost all BBC engines today came out of light trucks odds are you have a 402 or a 454 motor. Since few were around from 1984 odds favor a 454.
Big Dave
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