Whats the weight difference between a BB with Iron heads and water pump housing and a all Iron Small block?

Both with aluminum intakes......

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.

The RB blocks are in the 220-230 lb range. Don't know about the SB,maybe knowing the BB weight will help you some.

A bare Smallblock weighs about 165 +/- with caps.

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.

My .020 over bare 340 block is showing 160# on my bathroom scale.

Don't forget that if you run a small block and an auto you can run a 904 and save another 50#