Look for the casting numbers on the block. They are located behind the left head on the flange where the bellhousing bolts up. If it is a 3970014 casting, it is a regular 2-bolt main. This is the most common of the 350 castings. If it is 3970010, there is a 50% chance that it is a 4-bolt. If it is a 3970010, look for a small oil plug on the front of the block, near the top center between the base of the intake manifold and the top of the water pump. If that little plug is there, it's a 75% chance it's a 4-bolt. As Mooneye said, though, you can't be 100% sure until you pull the pan off. There are other casting numbers that have 4-bolt mains also. If the casting numbers begin with 14 and end with 776, 777, 778, or if it says 5.0L next to the casting number, it's a 305...

A 2-bolt block is good for occasional forays into the 6,000 rpm range and are plenty good enough for the street. Most of the time we are loafing around at 3500 or less. If you are going to thrash it frequently at the drag strip and consistently twist it up above 6500, then you need a 4-bolt.