Thats where my confusion was... the fact that the heads are "D" port and the headers are square portSince you have Vortec heads... I would go with the 760-66121. (They specifically mention "Vortec".)
I think I used the 760-66011 for standard old-school small block heads.
It's been a decade... I would have to search my project thread and maybe come up with a number which won't be specific for your application.
Hop to it, Harry!
Iola only is two weeks away!