This is an age long discussion, even before the huge increase of everything car related. I recall thinking 3.5k for a 6 speed was insane. Yet now I look back and wish I had done that over buying an R154.
For me personally, any options would have to consider; what happens if the gearbox fails.
Clutches, shafts, bellhousings, are never going to be as costly as a gearbox. Any conversion should have a decent supply of replacement boxes. I think the 350z option has been done but I haven't seen a huge amount of people solve this problem. BMW gearbox swaps were popular, but the 6 speeds (think M3 ones are also getrag) aren't disposable like the diesel gearboxs, but they have not so accomodating ratios.
I'm planning to sell my R154 after mines back on the road early 2019 and go down the BMW M3 getrag route.