The cranks are cast originally. I have a 440/4spd truck, when I bought the 77 440 core from the original owner, it had the original flywheel with the 3 big balancing holes drilled in the back for the external balance and was also drilled for the pilot bushing.. The flywheel was unusable, bought another from Brewers 4spd parts. The donor parts truck was missing its 4spd bellhousing, so I found another guy wrecking a 78 400/4sp truck and bought his bellhousing. His 400 engine had the same flywheel.
The only truck big block I ever saw with a forged crank was a 413 my friend bought from from some late 60's truck.