light weight big block
http://www.callies.com/crankshafts/magnum-xl/magnum-xl-440-and-hemi-mopar/
you probably want aluminum rods and custom pistons as well.
all depends on what you are planning. bob weights etc. rpm ranges. if you call them they may be able to give you all the info you need.
you can also message AndyF over on moparts.org he knows a lot about building custom high-perf high-end big blocks & lightened rotating assemblies. he might suggest some vendors
like hemi denny said you could also get a world products or keith black block. i know KB will cut you exactly what you want.