I believe the weight difference is almost 100-105lbs for the iron block. People go around and around on this, but I noticed that number by a trustworthy shop a little bit ago. I should have checked the ones to come through my hands, but never bother. Anyway under 800whp or so it's not a big deal. Lots of guys using the stock 5.7 aluminum blocks at 1000+, but the sleeve is at risk and the mains not as stable. I don't really care for all the "stance" lowering but it is a nice ride for sure.