Help id GM Big Block please

I have a BB in my 72 GMC C2500 pickup. It's supposed to have the 402 but I don't think so. What I can get off the back drivers side casting is: 82406. I can read it very well but the closest I can find online is 3902406 which identifies it as a 67 396 out of a camaro or chevelle.

On the front of the passenger side block is stamped: TI003IV & 7(0)4258(6)1 or may be 7(3)4258(8)1. I added the parentheses to emphasize the numbers I can't read clearly.

What have I got here?

Thanks in advance

'65-'68 396 325hp TH auto