As for info on old Kei Jidosha, I have been engaged in similar efforts. With the help of a Japanese dictionary and cross referencing on ja.wiki, I have been able to derive almost all the information I want from the following:
360cc Light Commercial Truck 1950-1975. ISBN: 9784861441394
360cc: Nippon Kei Car Memorial 1950-1975. ISBN: 974861440830
Car Graphic: Car Archive Vol. 1, The 60s. ISBN: 9784544091717
Car Graphic: Car Archive Vol. 5, The 70s. ISBN: 9784544091755
Car Graphic: Car Archive Vol. 11, The 80s. ISBN: 9784544910186
Chris Rees, Microcar Mania: The Definitive History of the Small Car. ISBN: 9781870519182
Kazuo Ozeki, Suzuki Story. ISBN: 9784895225038
Kazuo Ozeki, Daihatsu. ISBN: 9784895225052
Kazuo Ozeki, 360cc: Memories of Japanese K-Cars 1951-1975. ISBN: 9784895225014
Old catalogs and such are naturally priceless, as are period car books such as the Italian "World Cars" or the German "Auto Katalog". I included all the ISBNs, these books shouldn't be very hard to find via Amazon.co.jp or, even better, if there is a Kinokuniya Books store in your town. Good luck with your project, hope it is something we may see when you're done.
Just thought I'd update this in case anybody's searching it. I ended up getting this book:
The complete history of the japanese car 1907 to the present by marco ruiz. Published in like '87 so it only goes up to then. Got it for $8 shipped. Great deal.