This is getting off subject but scrappys dont pay 200 dollars for a ton of scrap its around the 80-100 dollar mark, unless its worth double wherever your from. full stop.
And you will have trouble giving an rb 30 away. i know i have had many and they like skylines aren't worth diddly squat.
And try finding a r31 skyline in the wreckers that has a good diff in it, they are usually pintaras and the diff is well worn as pintaras diffs always ended up being noisy, wearing the crown wheel and pinion and being 25 spline are only worth the gears anyhow if they aren't worn.
See, thing is, i have actually looked for these things and up here in qld at least, fact is there are plenty of pintaras around, usually with stuffed diffs( they had huge problems and recalls to fix the things under warranty) but it is quite uncommon to find a r31 skyline diff that hasn't been abused and maybe 1 out of 10 at the wreckers are actually skylines, not pntaras.
I think you may be incorrect in saying there are 3.7 gears available, no vl ever was made with these gears and factory literature make no mention of skylines ever having these, even as an option. I will have a chat to a former nissan mechanic tomorrow who i work with to confirm this, if you like so you can know once and for all.
ANYHOW, I still have this diff available if anyone is interested in buying it. I want 400 dollars for it, and if you want to find a 250 dollar skyline on ebay, within a reasonable distance and convince the owner to let you pull the diff out at his/her place and then try and convince some srcappy to give you more than 50 bucks for it, go for gold! All the best to you, i wish you luck.
If you want this diff you can come and pick it up, take it home or to a workshop and start doing what needs to be done to put it in you datsun.