Here's the problem:
If you have not owned the motor ever since it was produced on the Chevrolet assembly line, then you have no idea what has been done to it by its previous owners. The decks could have been cut already or the last builder could have used those fosdick rebuilder pistons with a 1.540" compression height that puts the piston crown down in the bore by another 0.020", wrecking any chance of getting a good squish/quench.
In the first place, these motors are mass produced and although they have a set of standards, plus and minus, the block deck height could be anywhere from (for instance) ~9.015" to ~9.035". All it takes is a small chip in the register of the mill that the operator failed to clean out from the previous block that was cut in the mill to make a heck of a difference. The other thing about milling the block at the factory is that you have no idea if both banks were machined the same from the centerline of the main bearing bore. Matter of fact, is you find a block that has the same centerline to deck dimensions on all four corners, you should run right out and buy a lottery ticket. If the block is milled askew, then you would have 8 different static compression ratios, a different one for each cylinder.
That's the whole idea of building a motor. You use blueprint dimensions from the factory or other sources to equalize everything, so that A works with B and B works with C and D. First, you inspect the main bearing bore for all holes being round and parallel with each other. If they are not, you correct them by the align honing or align boring process. Once the mains are valid, you measure the block deck height at all four corners with a 12" dial caliper. If the decks are all the same and the block deck height falls into place for the stack of parts you will be using, fine, build it. But is the decks are off, like I said, you will have 8 different static compression ratios, the heads will sit askew on the block and the intake manifold doesn't have a chance of sealing up properly on the heads.
The least a fellow can do is to pull the heads and measure the piston deck height. That will tell you what gasket you should use, or if the piston deck height is excessive, or the measurements are off from the front to the rear of the block, it will tell you that the motor should come apart to cut the decks so that you can engineer a proper squish into the motor and to have the same static compression ratio on all 8 holes.