The 305 and the 350 connecting rods have the same dimensions which are for the 2.100 crank pin and are 5.700" long, but over the years there have been a lot of different rods and they don't all weigh the same. There are some of the higher performance 350 rods (GM performance powdered rods) that are heavier and stronger.
If you are not going to have the engine balanced with the heavier pistons, then I wouldn't worry about the difference in the connecting rods. But, you do need to check TDC to verify that the mark on the balancer lines up with the timing mark pointer.
Bill