Quench is basically the distance between the top of the piston and the bottom of the head. In a SB chevy you usually want the quench around .040 . First measure how far the piston is from the top of the block when it is at TDC. For example, lets say it is .020 in the hole. Then you need to find a set of head gasket around .020 thick. That would give you a squish distance of around .040 which is just about Ideal. Most recommend not going below .040 to be safe or above .050 to still get a good squish of the mixture. You need good rods and rings to run tight clearances though. If the rods stretch during high rpm or the pistons rock to much, the piston can hit the bottom of the head.

Here is a brief article explaining it a little more.
Chevy High Performance

-Dan

 

Most of the better aftermarket gaskets are about.040 when torqued so if you run the piston a couple thousanths in the hole you get .040 with steel rods.

 

Quench is important because it really helps get the fuel into the chamber. If the quench area is big, it becomes a dead area and can take away power. I have run quench numbers way up with no drop off in power. IMO, it depends on the combination. Some engines are much more prone to detonation and spark knock with a large quench. As for the compression, adding a small amount will net very little in power. This is especially true as compression is increased. If your deck height is way off from end to end, you'll improve power just by making each cylinder closer to the same compression. I just did a 454 for a guy where the decks were .014 to .022 from the factory.

 

they answered most of your questions but the one thing i want to add is, dont mill your afr's untill you call afr. they are setup a certain way for flow numbers and combustion shape. by milling those heads it could hurt performance. if you contact afr they will help.