http://www.nastyz28.com/sbchevy/sblock.php
http://www.nastyz28.com/gm-chevy-cod...s-suffix-2.php

Take a look at those two links I provided. You will need to pull the numbers at the front of the block on the flat plate on the passenger side to really know anymore. Most likely it's a mid 70's 350 2 or 4 bolt main. Unless you plan to push some SERIOUS horsepower and torque I wouldn't worry about the mains. 2 bolt is plenty for a hopped up street motor, plus they make kits to beef a 2 bolt main up. Your bigger cars and trucks I believe are the ones that will have the 4 bolt main. Only way to REALLY know is to pull the pan and check. Seems like EVERYONE online will sell a 350 and tell you it's a 4 bolt main and then when you pull the pan, you find out it's a 2. Lots of people blow lots of smoke to make things sound better (just like the whole double hump head crap).

Good luck and I hope the links shed some light on your questions!