As much as I hate the taste of shoe leather (see above), I'll still go out on a short limb and say this car has a SERIOUS identity crisis. I lean toward the "totaled '85 GN chassis with an '86 5.0L body shell" theory. Someone, somewhere created this thing, and I doubt very, very seriously that it was General Motors. If it was totalled as an fairly new '85 that person should've swapped V.I.N. tags while he/she was swapping everything else.
The eigth digit in the V.I.N. is a "6". I couldn't find a reference to that at all on the GN/T-type page. Did I miss it or does someone know what it signifies?