I did see a video of a machine welding 2 shafts together, but they were pre-heated and slow-cooled and re-hardened after, if you just MIG it at home, they break right next to the weld. (Seen that happen so many times..)
(Machined from blank steel-) Custom shafts are not cheap, but theres companies that make them, some examples are already in this topic.
its the best way if you have the money.
I'll be facing this dillemma soon, as my diesel gearbox will probably not survive long with the added torque a turbo, there's however a drop-in replacement gearbox that matches the engine bolts, clutch, flywheel, everything, but theres no axles that'll match my car to that gearbox.
I heard rumors that certain axles would fit if I take the wheel hubs from that car and have them machined so they'll fit my wheel bearings.. We'll see.
But that brings me back on topic, the splines on the MGR side are not just the same ones Toyota puts on everything? that would make it somewhat easier perhaps, to find the correct length shafts. Machining down a set of Toyota wheel hubs to fit your wheel bearings is probably cheaper than custom shafts.