The crank is one piece, forged steel.
It looks really strong, it has huge journals, the same bearings are used for all the V6 engines, from 305 to 478, and the V12, and I believe the 637 v8 (it's also in the v6 family, as it is 60 degrees and essentially a 478 with two more cylinders and a balance shaft).
Again, the most info on the net about the V12's can be found here
http://www.6066gmcguy.org/TwinSix.htm Grigg
