I don't remember where I saw it.... but what I saw was that the b-block was not much lighter than an rb block.
Crank and rods can be lighter, the 400 forged pistons I had were very heavy.
So, I would assume the majority of the savings will be in the heads and intake.
My guess would be around 60-80 lbs, leaning to the lesser end of that.
(Aluminum rat heads save that over the iron versions in the heads alone)
