You have to keep in mind when figuring value that ANY hard to find component of the motor can be had, but the block is only one of one to your car . If you have the original block in your car and everything else is incorrect,that is better than having a 100% show car correct complete engine with a NOM block, even though all the correct components will be more expensive to repalce than the block[if you are lucky enough to find it]. If any Z motor has a market value of 6K, than its worth alot more than that if its yours. If you compare 2 zs that are similar,except one is factory block and the other repalced or decked , you are going to find a much bigger difference in market value than 5K...