I haven't found any one now making Ductile iron caps any more. Mopar stopped making and selling them and Milidon doesn't offer them either any more
Milidon does offer steel caps, so does 440 Source, but I'm leery of using steel caps on a cast grey iron block
I have one of the Koleno high nickel blocks that I am planning on using soon for all out N/A build, hopefully the main caps on it will work for my power level
I wish Dart, Brodix or some other good quality manufacturer would make a good Mopar V8 race block out of cast Compact Grey Iron like they do for our competition in the Chevy and Ford camps