What seems to be the average in the market for the last year is:
914-6 = $1,200.00 restored
914-6/GT = $1,500 restored
914-6 = $6-800 unrestored (with pistons because you can't find pistons anymore)
Ironically, if you could find a Ferrari guy in need, they would probably sell for $2,500. No eyes batted.
The spacers and the longer fasteners are the only difference between the -6 and the -6/GT calipers. Pistons are all the same (larger) 38mm size).