All so, if you get into the Gen V or Gen VI big blocks you wil have a few differences. Gen V does not have the mechanical fuel pump boss on the block, and Mark IV production heads most of the time leak at the deck coolant holes.(The Gen V deck coolant holes are shaped much different than Mark IV), and the Gen V production blocks do not have a boss for the clutch pivot ball, to hold the z shaft for the clutch. The Gen V and Gen IV also went to the one piece rear main seal with its' attending oil pan,crank and seal. There are some features that get mixed and matched when you go to a GM bowtie block so be aware of what you want in a block. Some good features of the newer blocks are the ready to go hydraulic roller cam design built into the block and all are 4 bolts mains. A net lash valve train and having the right flex plate or flywheel are just a few of the little things to keep in mind when building the new engines, but the net lash valve train is easily converted over to an adjustable system.