Dart big M blocks require +.300 taller lifters. Everyone is carying them, and they are listed quite often in the books. In fact, Crower lists every part number with and without the .300 taller lifter option. Some people are starting to make all of their lifters taller, and sell that same one for the +.300 blocks and standard blocks.
They are a great lifter. As good as about any on the market. Lots of the crate engines from guys like Sonny and Reher Morrison use them almost exclusively.
But if you can get them for cheap, make sure you get the +.300 and get em.