Crankshaft counterweighting

whats people thoughts on when you should go to a fully counterweighted crank. Eg big power ls engine is it more a rpm thing or hp or a combination of both?.

A fully counterweighted crank improves stiffness which is important for bearing life at high rpm or high power levels. the downside is that the fully counter weighted crank weighs more and hence has a higher mass moment of inertia (MMOI). So really it's going to come down to what you're trying to achieve in terms of your build. There's plenty of LS engine's making 1000+ hp on a stock 6 counterweight crank.

I’m looking at 1800hp fwhp, my engine machinest thinks the Callie’s dragon slayer will do the job. But I noticed there is a dart fully counterweighted crank now, he’s not sure on that one because he’s never had any experience with them.

At 1800 whp you're going to need everything going in your favour and I'd certainly be suggesting a CCW crankshaft. I can't advise between the Callies and Dart sorry as I've not used either. Both are reputable companies with good reputations though. I'd do some research on LS groups and see if there is a clear cut winner. I'm guessing at this power level you're not planning on a stock block, however I'll just mention that a CCW crank won't fit a stock block without some machining work.