The overdrive would be nice for street driving, granted, as would the bigger and stronger components of the RE403 over the Skyline 401 but like you said, they're much more expensive to build to take the power being suggested than an equivalent C4 or TH400 which are known quantities where spares are very readily available.
If it came to it you could get Mike from MV to put a JATCO together as he does some good work at reasonable prices but the standard Jap boxes are too hit and miss when you start leaning on them hard. I'd stick with something that you absolutely KNOW will do the job time after time.