What is the difference in a 305 and a 350?

The 305 & 350 are basically the same engine. The 305 just has a smaller bore(3.736 vs. 4.000), not quite the power or torque, but durability should be almost identical. It is more costy to rebuild a 305 than a 350 due to the popularity of the 350. If power is not an issue then go for the 305 if you can get one cheaper. If you want more power, then a 350 is the way to go.