With a little more digging the issue with F8 manifold clearance has already been brought up here previously in Jim Prices thread “Exhaust Manifold - Right Side - F8”
http://www.mapleleafup.net/forums/sh...ad.php?t=11790
I will theorize the original manifold “78-9430” which is mounted flat against the block had a heat soak, expansion and contraction issue. This caused cracking and would be replaced by what was readily available through post war years.
With the 15/30/60 having larger numbers and longer production run. Its purpose built chassis had more clearance the issue was never noticed, allowing the use of 21-9430 car manifold.
With the F8 being such a short run and based on the civilian chassis there have been a few other oddities I’ve come across. Including the steering, suspension and brakes. The exhaust clearance in some ways isn’t a surprise.