There is virtually no way to tell how much was decked off the block. The factory specs are out to lunch in the real world. I have seen them more than .050" over max spec and some just about that much under. Sometimes on the same block and sometimes from end to end on either side.

So If the thing is parallel and the pistons are below deck, you are in the ball park.

Also remember that stock replacement (rebuilders) pistons are built .020" short from the manufacturer.

I would not really worry about pushrod length on this stock type motor unless you are going to use a .480" or more lift cam. Even that much cam lift on stock 882's would be a waste.

Stock SBC 400 rockers are 1.5 ratio not 1.7 (366-454 big block). You will not have excessive compression ratio with dished pistons and 75 cc heads.