You need a dedicated one pieces shaft ideally 1” shorter for the getrag. You will also need a front shaft also from the R34 for the transfer box to the front diff.
Nitto are not the authoritative answer on oil pumps or spline drives. Can respect their own findings maybe it wasn’t so successful based on what clearances and spline profile they tested with. We carried out our own R&D prior to launching them in 2014. We have successfully been selling spline drive kits for almost 5 years now with over 900 kits sold and yet to see the “Nitto theory” come to life..... Ironically our Jun kit fits the Nitto pump. Have a good few customers running a Nitto pump with a spline kit. That in my view is an anwesome pump because Nitto did use better materials and a billet 7075 back plate compared to Jun/Greddy.
If you have a Tomei Pump you do not need to scrap it. We supply a full conversion to Spline Drive. You would be better off with a Tomei as it it a better overal Pump.
They have in some cases become oval and caused binding and lock up of the oil pump. Nothing to do with Spline Drive. We do however have Spline Drive upgrades for most pumps and the theory they bind due to crank flex is just a myth. Both ourselves Supertec and Spool have been selling these for the past 4 years. Zero gear failures reported to date for our kits.
Spoke to Getrag themselves and they insist on using Dexron II oil because these gearboxes were designed with bushing that do not suit modern synthetic oils.
I am now going to use Toyota T IV.