The 1:1 scale kit - manufactured by Tamiya's industrial forerunners! (What were the WWII Japanese shipbuilding companies - or was everything just built under IJN BuShips?)
Seriously though, I wouldn't know who did a good Shinano conversion - I don't even know what Shinano really looked like, in detail. The island, for example: I'll bet a whole lot of models - like my ancient Hasegawa 1:450 beast - probably got significant features wrong/missing entirely.
I would love to post a good set of plans and historical photos for Shinano here on ModelFleet - if you (or anyone out there) are able to find anything(s) good and can upload 'em - or even just send 'em to me, and I'll create the whole post.