The numbers are on the right side (facing engine) behind the distributor next to the distibutor shaft. I have not been able to locate any other numbers on the block.
The sources that I've tried so far are a couple of older Hollander Exchange manuals (a tip for anyone else in need)that have sectons on motor numbers. Nothing that begins with HAA is showing up there, but they do not include #'s for GMC's.
Anyway, any help will be appreciated!