A CNV suffix code has only one application 1971 Impala 250 hp 350ci 2bbl carb with a TH350 trans you should be able to measure the bore at 4"
two answers so far 70 and 71 http://www.nastyz28.com/chevy-engine-code-stampings.php http://www.nastyz28.com/sbchevy/sblock.php
There was nothing here when I started looking so I am posting any way http://www.nastyz28.com/gm-chevy-codes/chevy-engine-codes-suffix-3.php