I don't know the specific answer but here are some general tips. Hope it helps


* The japan-built R31 diff uses a Nissan flange, while the aus-built R31 diff has a six-bolt flange. Most use a 2-piece shaft (two tubes, three joints)


> Is there a shaft that can be shortened?
Since both parts you are using (gearbox and diff) come out of skylines, try asking the question at r31skylineclub.com. People are always asking about making one-piece shafts. Maybe a bluebird 910 wagon or something might work
* The stock 1200 shaft is 1334 mm overall length, and it would need to be shortened to fit the longer R31 diff head
* R31 shaft from what I understand is alreay too short for the 1200


* Many shafts cannot be shortened due to the way the are constructed at the factory. So what shops do is use a new shaft and transer only the input yoke and output flange to the new shaft (making cost of shortening nearly same as a custom shaft)

> Am I better to just get a custom one made
Maybe. Shop around, because different shops will charge vastly different amounts. In my part of the world, from $150 for shortening up to $500 for the same thing! New custom shaft prices seem more consistent $300-500