http://www.arengineering.com/articles/sonicbig.html....15 lbs difference between a 383 and 440 shortblock.
basically, you might find a 440 that is 30 lbs heavier than a particular 383 and you might also find one that is almost identical in weight......certainly a negligeable difference in any case and one that is more than offset by the difference in displacement. A change to headers or an aluminum intake is more significant than the difference in the block weights.
Dave