This is from the full book, CHEVY BIG-BLOCK ENGINE PARTS INTERCHANGE. For a comprehensive guide on this entire subject you can visit this link:

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Unless you require a specific block casting for a numbers-matching restoration project, there is no longer any performance or budgetary reason to seek, build, or hoard original Mark IV engine blocks. Newer Gen VI engines came only in trucks; blocks have become more available in wrecking yards. Chevrolet Performance offers all-new blocks that provide all the improvements necessary for top performance in both street and racing applications.

The following features make these modern blocks far more attractive than original-style Mark IV blocks.

1. Standard production four-bolt mains
2. Priority main oiling
3. One-piece rear main seal
4. Thicker deck surfaces
5. Thicker cylinder walls
6. Revised coolant passages
7. Accepts all Mark series 
cylinder heads
8. Machined fuel pump pad
9. Revised oiling to allow for bigger cam bearings/cam lift
10. Accept roller cams or flat-tappet cams 

Plus:
• Bowtie Sportsman versions have splayed-bolt main caps
• Choice of siamesed or open cylinder bores
• Can be drilled for use with ten-bolt front timing cover
• Auxiliary oil pressure line added to front of block
• Racing-style oil filter cast feature with added oil pressure port
• Two machined bosses added for manual transmission clutch pivot
• Material added around lifter bosses
• Additional clearance for roller timing chains

Late-Model Blocks

All Gen VI blocks have four-bolt mains, priority main oiling, and leak resistant one-piece rear main seals. These cylinder cases have revised cylinders in either siamesed or open configuration and they provide thicker cylinder wall and deck surfaces for greater stability. They accept all common big-block cylinder heads and they incorporate a fully machined fuel pump pad and provisions for either hydraulic or flat-tappet lifters.

Gen VI cylinder blocks offer an extensive resume of built-in high-performance features that are generally found lacking in Mark IV and Gen V engine blocks. 

Gen V and VI blocks have the engine size cast into the block just below the deck surface. The oil filter pad is flush with the oil pan rail instead of recessed as in Mark IV blocks. The oil cooler ports are positioned on an extension of the pan rail (except on Gen V Bowtie blocks) and the freeze plugs are 1/8 inch smaller than those used on Mark IV blocks. (Photo Courtesy Scoggin-Dickey Parts Center)

Written by John Baechtel and posted with permission of CarTech Books

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