It will depend on where the cam lobes are when the piston is at TDC for its particular stroke. That is up to the cam grinder. Pontiac chose the original firing order.
At every "firing" position, two pistons are at TDC: 1 & 6 then 8 & 5 then 4 & 7 and finally 3 & 2. You can verify this with a short block. The firing order is ultimately dictated by the cam lobes. The power, smoothness and health of the crank are the result of the chosen firing order. Check the scavenging result: Odd, even, odd, even, even, odd ... it simply moves the two same-side pairs around, but it has an effect on the torque curve.
I recall some circle track and endurance racing groups playing with the firing orders back in the 70's and 80's to try to squeeze more out of the engines.
It was also common on reverse-rotation boat motors to alter the firing order to smooth out the torque ripple at lower RPMs, if I recall correctly.
