B310GX
I am well, but i think i have bad news for you.
According to my parts man, who had been at the dealership dealing with Datsun stuff since 1000's were new, the 3.7 to 1 ratio was used in the last of the Sunny wagons that were fitted with the A15 engine. It would seem that the A14 powereed ones had 3.9 to 1 ratio diffs.

If i can come up with a factory Sunny workshop manual, it should give us the good oil.

I would look foward to hearing what you find.
I do not know it there is some sort of code on the third member that discloses the ratio within, but i hope so. Can anyone help with this.

I am surprised about the 2mm spacer as Stirlingmac had not mentioned it, yet he runs one in his 1200 coupe.

If I go with plan "B" & run the CA16DE, with OD 5 speed & the 3.7 diff, it will be one very nice & economical cruiser, & cruisin' is where it's at [for me] With a 7.9 gallon tank @ 40mpg i should get a comfortable 275 to 300 miles between pit stops. [About450 to 480k] Thats my style of driving.