...These are 906 Magnesium Intake Manifolds and differ slightly than those used on the 911R.

See the top flange of this 906 intake manifolds https://pbase.com/9146gt/image/31433108 when compared to the 911R https://pbase.com/9146gt/image/32228077, the way the linkage mounts on the mid-engine 906 is much different than the linkage configuration on the 911R and 914-6 GT.