Ok, from the beginning, the 305 is the worst performing Chevrolet ever. The miniscule bore shrouds the intake valves, the valves are tiny as are the ports in the heads. The rod length to stroke is way out of whack. The engine has the 400 CI crank journals (big) and about 7.8:1 compression. The best thing to do is keep the accessory drive, distributor, water pump and oil pan. Donate the rest to your local high school shop class (taking the tax deduction) and build the 350.
The difference between Chevrolet 350 and 305 is in bore and stroke. A 305 has a 3.740 bore and 3.48 stroke while a 350 has 4.000 bore and 3.48 stroke. Overall, there no much difference between the two cars.