I was hoping for a slightly taller rear end for my 320 with J-13 installed. I pulled the carrier on the parts truck, and I think it may be from a 520 or 521, but not sure. It has the finer axle splines of the later trucks, not the coarse Acme splines, but the gear ratio is 39:8

http://farm3.static.flickr.com/2316/251 ... e5be_o.jpg

http://farm4.static.flickr.com/3054/251 ... 99ef86.jpg

Any thoughts on this. If the gears check out, can I just swap out the carrier and the axles/flanges? Then again, there's no advantage to doing that with the 4.875:1 ratio, it might be the same. Did the 320s go to the finer splines in the last months of production?

If you have a stock 320, that would be the same ratio that's in it now.  That is, unless your're one of the few really lucky(!) ones that got the 5.143.  What ratio are you looking for?  520's had the same 4.875 as the 320.  521's had a 4.375.  Other rear ends that can be used are the 3.889 from the 1600 Roadster and the 3.70 from the 2000 roadster but that might be tough for a stock J13.  There are other differentials that will work like the 3.70 from the 810/Maxima wagons but they are metric.  Still do-able but require a little more work.  They'll all require a change of side gears to work in a 320 and you'd use the stock 320 axles.  I have a list but I can't find it right now. 

What I was hoping for was to send the carrier to Sloan Racing for the 3.889 swap. Bit since the parts car has the small splines, and the white truck has the big splines, I don't have the proper side gears for a direct swap-out. I use the white truck for an errand runner, so I want to minimize down time if possible. The 4.375 ratio would work too.

Which ever H190 differential you end up using will require the use of the 320 side gears and 320 axles in order to use it in a 320.  When I did that swap on my 320, I recall that the 320 axles were shorter than the later 520, 521, 620 etc.   Check with Tania (vindats on this site).  Maybe she has a set of 320 side gears that you could send to the shop along with the differential.