I got the opportunity to measure the smallest internal diameter on my wheel today. As indicated in the picture below, it's approximately 390mm measured from the front.
The diameter does taper out behind where I measured this diameter, but from what I can feel with my finger, only by about another 10-15mm increase in radius.
With reference to the Mov'it measurement guide, in order to fit a 370mm diameter 4-pot kit I need 418mm internal diameter to accomodate the disc and caliper. I'm not confident that I have this much space inside the wheel... it would be a very close fit!
I'm therefore convincing myself that the 342mm kit will suffice and that 370mm would be overkill anyway. The largest disc size I've seen fitted to a Ghibli is around 330mm. Generally, the largest I've seen fitted to a 3200 GT is 350mm. Bearing in mind that the 3200 GT weighs about 300kg more than the Ghibli, fitting the 342mm kit should provide more than sufficient braking power! In terms of aesthetics, it should look fine too... you'll just be able to see the full caliper through the spokes, where with the 370mm discs the caliper would be slightly hidden behind the rim.
In terms of axial clearance, I measured at least 80mm clearance between the spokes on the wheel and the current disc, so there shouldn't be any issues there.
Getting technical... the disc braking area currently on my car is approximately 42,312mm2 (300mm by 190mm disc diameter). With the new disc, the braking area is approximately 63,478mm2 (347mm by 190mm disc diameter). Therfore, the effective increase in disc braking area is 50%. I think that's good enough!