As this thread unceremoniously died about 4 years ago, I assume no IJN sub plans were found. Too bad, the Kaidai Type VIa would be a great subject.
For anyone interested, I did find plans for two IJN subs.
IJN I-400 series (aircraft carrying sub):
I can't vouch for the accuracy, but the drawing includes cross-sections that are probably sufficient enough for the basis of a hull.
IJN I-201 series:
This is from Wikipedia, so I can't vouch for the accuracy of this drawing either. There are only a few cross sections/bulkheads, so I am not sure this is adequate for making a hull mould or not.
In the books on IJN subs I've seen, there are no detailed drawings. I suspect, short of serious research in Japan, the best option is making an impression from a scale plastic model and scaling it up.
If anyone else has a source for IJN subs, I'd be very interested in learning more.
Upon further research, I came across some plans for a 1/48th scale IJN Junsen Type B submarine:
At that scale, the hull is 89 inches long!!
In fact, a builder at rc-sub.com put several years into it, before selling the kit. He had a web log page on his work up to the sale:
This build was based off of a kit from ScaleShipyard.com:
At US$595, and then US$195 shipping (ground lower 48 states), the initial investment is significant, but the fiberglass project looks very nice.