I was going to do a cast crank 451 build just to sort out these questions for myself but I never got around to it. I'm pretty sure that if you use lightweight rods and pistons you can get it to balance internally without any heavy metal. That allows you to use the good dampers and flexplates. You can also just add weight at one end if you need to, for instance, use a neutral damper and an offset balanced flexplate.

The journals need to be cut and the counterweights need to be cut down to clear the main webs with either a forged or a cast crank. Depending on what kind of shop capability you have in your local area it might be cheaper to buy a new 451 crank than rework an existing 440 crank. It all just depends on the situation and the size of the wallet in question.