I have heard that it's in the 75-100 lb range. But I never weighed them myself.
A low deck big block (361/383/400) would be a little lighter than a raided block BB (413/426/440. They did make a raised block 383 about 45+ years ago, but not many.
A B motor should weigh about 20# lighter than an RB (imagine 2 cast iron deck plates .75" thick which is roughly the difference between the blocks), but it's not always the case as the early 400 B's are generally a bit thicker inthe main webs.