I'm certainly NOT an expert, but my understanding (from reading the GMPP catalog) is that the Mark IV is the first generation PRODUCTION Chevy big-block, and that it was succeeded by the Generation V and then the Generation VI versions.
The Mark IV had the main oil gallery located in the lower portion of the block, abover the oil pan rail. The problem with this location is that if you wanted to use a longer stroke crank and needed to machine the pan rails for clearance, there was a good chance of cutting into the oil gallery. A BAD thing.
The Gen V fixed that by moving the gallery to a more "benign" location.
The Mark IV also used a two-piece rear main seal, which was prone to developing leaks. The Gen V incorporated a one-piece rear main seal, thus greatly reducing leaks.
With the Gen V, Chevy also introduced the siamesed-bore 502, which had no cooling jackets between the cylinders. The thicker cylinder walls, however, allowed larger bores without weakening the cylinder walls.
I understand that the cooling jackets in Gen V blocks were shaped differently than those in Mark IV blocks, which made using Mark IV cylinder heads on Gen V blocks a NO-NO, because coolant would seep into the lifter valley because the head jackets would not mate-up properly with the block jackets.
One change that was controversial was the elimination of the mechanical fuel pump boss on Gen V blocks. This meant that owners of older, mechanical fuel pump cars had to switch to an electrical fuel pump, which was something many folks did not want to do.
I'm sure there were other updates incorporated with the Gen V, but I can't think of any.
With the Gen VI, Chevy put the mechanical fuel pump boss BACK on the 454/427 blocks, but continues to omit it on the 502s.
Additionally, the Gen VI block is machined for the use of the "magical" factory hydraulic roller-lifter cam.
Beyond these changes, I'm not sure what other differences there are between the Mark IV, Gen V and Gen VI engines.
Perhaps others can fill in any gaps I've left.
Big O Dave
[This message has been edited by Big O Dave (edited 05-07-2000).]