I just picked up a 71 Malibu. The motor is an 8 cylinder with a 4 barrel Rochester carb. How do I determine for sure the motor is a 350 versus 307? Where would I find the block number and verify the engine displacement?I was told the motor is a 350 but the seller is not reliable. Either way the price was right.

The block casting number is right behind the drivers side head. You should be able to see it with a flashlight just looking down at the engine. Look at this site:

http://www.mortec.com/castnum.htm

Also, on the front of the block there is a pad with stamped numbers. These will tell you what the engine is and what vehicle it was originally installed in. Look here, scroll down the page a bit for pictures:

http://www.nastyz28.com/chevy-engine-code-stampings.php

I dont know the measurement but the 307 dampner is noticeably thinner.

Locations that Bill referred to
http://www.yearone.com/updatedsinglepages/Id_info/gm_engine_id/gmidimages/endinedetail1.jpg