Ford Racing lists a 75-100 lb difference in shipping weight between their 302 and 351 based crate motors. I'd think most of that is in the block and rotating assembly.
Sort of - a 351 with AL heads, ministarter and the tubular GT40 upper probably weighs about the same as a 302 with cast iron heads, a cast upper intake and the bigger starter. But put the same components on the 302 - as a lot of modded motors are equipped - and you still have the 75-100 lb. difference.