I believe that is because w2 headers had 2 different bolt patterns if I remember correctly if you hold up a standard exhaust gasket the ports don't line up.
but its been a while since I was playing with w2s

70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4