SBC 406 CID, It was an original 400 when I got it, I bored it .030.

Bore 4.156
Flat Top pistons
Fel-pro .039 head gasket
World Sportman II heads 64cc Chamber

I got these numbers from a SBC Chevy book

400 CID Basic Dimensions
Stroke 3.75
Deck Height 9.025
Deck clearance .025
Rod Length 5.565
Compression height 1.56

10.82-1
http://www.wallaceracing.com/cr_test2.php

there are other things you need to know...such as if the block has been decked. And if it has or hasn't been, how far down in the hole are the piston crowns at top dead center? It would help to know what pistons you have and how many cc's the valve reliefs are too. Most aftermarket flat top SBC pistons have 6cc valve reliefs.

The block has not been decked to my knowledge. I pulled it out of a 74 Impala the owner said the engine was in its original state.

I assumed, 6cc valve reliefs, .025" in the hole and came up with the 10.82-1 CR.
Actual chamber size, valve relief volume, clearence from the top ring to piston top along with actual deck height would be needed for a true compression ratio. A good machine shop can pour the mentioned things and come up with an accurate measurement.

You'll need to know the bore size ( I don't think that you listed that) and if you don't know the head gasket bore size, then you might just type in a figure that's about .030" bigger than the actual bore size. I don't remember what a 400 CID block bore is with a .030" overbore (maybe 4.155"?).

if it's a stock block that hasn't been decked then the piston crowns probably sit about .025" in the hole unless the piston height (aka "compression height") is bigger than stock.