I don't remember the heads on my 305 v6 being that terrible heavy, but I was a lot younger back then and pretty damn strong (worked in the timber every day!). Also, I had pulled the head on my old 4-cylinder Cat and that thing was
really heavy!
Neither do I remember torque specs or order. I suspect that I just guessed back then, and the engine ran fine for years afterward, pulling heavy loads of logs. Those v6 GMC engines are pretty simple and tough, so as long as you don't do anything too stupid then you can't really hurt them. Too much rpm can be bad news, though.
Did you pull the distributor to remove the heads ??? You don't have to, you know
I never did.