This is a subject I have been interested in for years and yes, there seems to be little info beyond casting number differences. However, from years of gathering data about these things, here is what I can tell you; All 305 crankshafts are made of cast iron, so if yours is forged steel, it's a 350 crank. The counter weights on the throws are going to have different amounts of weight removed either by drilled holes or shortening of the weights. I only wish I had info on the 305 crank vs a 350 in photographic form to show you, but suffice to say, larger holes/more of them means balancing more weight. Since 305s use lighter pistons this should equate to a 350 crank unless the opposite is true. There have been stories for years about 305 rods being smaller or thinner than 350 rods but I doubt this is true. All small block rods are forged steel in construction and vary only in two ways; material and thicknesses, and hardware(such as different size bolts) except the short 400 rods which are unique. I doubt seriously when they were being forged there was one bit of difference because it would be cost prohibitive. This means the only real difference in the two rotating assemblies is the pistons, which are nearly .750" larger around on 350s than 305s so you can see how much weight is involved here. Generally speaking, if you plan to use it, identify it as best you can and use the appropriate harmonic dampener that should go with your assembly. I have heard of these cranks being used interchangeably but I'm not sure if any balancing is required. My books don't say so but this seems unlikely to me. A good experienced machinist will know what to tell you about that. I hope this information helps you.
