Your problem isn't how reliable (very reliable), but in finding one. The supply in Australia dried up long ago, with only the occasional private sale or wrecker having one. That's why Toyota gearboxes are used with an adaptor/bellhousing. Dellow do the most common kit, but people on this site have made their own using the bellhousing from a A series auto. So if you can find a 5 speed it'll go straight in & shouldn't cause any problems, especially if you fit new bearings first. But it might be cheaper & easier to get the Toyota box & the Dellow kit. Are you still using the A12 or is it an A14/15? The overdrive 5 speed really wont be much use around town, but is an advantage on the open road.