That is very interesting Ian! Perhaps the later Deon chassis that you have is wider to cope with the Ford engine? I guess that narrowing the track but keeping the same driveshafts would need checking to see that the shafts still had enough travel, lengthwise, so they don't bind. Given that the Deon body shell was originally moulded from a 206 Dino the 206 tracks are what you want to replicate? To my eyes the way the wheel/tyre combo fills the arch on the 246 looks better on the 246 than the 206.
The Ferrari Dino 206 was:
track front: 1425mm. (246GT is 1425)
track rear: 1400mm. (246GT is 1430)
here are a few images - the 206 was originally specced with narrower 185 tyres as well as having a narrower track in the rear:
206GT_track.jpg
206GT_rear_arch3.jpg
246GT_rear_arch.jpg