A second pair of hands might help.
Also it can be helpful to remove the rear coupler so that you (and maybe a helper) can stand the locomotive on its back (tender) so that G118X-00V05 is facing up while you are working on it. If you do this, be sure that someone is holding the loco firmly as it will fall over if it isn't supported!
Here are pictures of how I stabilized G118X-00V05 while putting the new driveshaft in:

OR: using a pair of Threading Tweezers for a Serger (sewing machine):

(the tweezers themselves:
And here is a picture of the inside of the (old, broken) assembly with both the yoke and the driveshaft inserted into their respective grooves:

The key is to stabilize G118X-00V05 while it is still attached to the center gearbox shaft. This is tricky and will take time. Before you stabilize, turn G118X-00V05 so that the screws are easily accessible while you're working. Don't apply too much pressure with the stabilizing shims as the bolster and the yoke are both old and fragile--only enough pressure to stabilize the bolster while you insert the shaft and then screw the bolster cover back on. It will take time and patience.
Also it can be helpful to remove the rear coupler so that you (and maybe a helper) can stand the locomotive on its back (tender) so that G118X-00V05 is facing up while you are working on it. If you do this, be sure that someone is holding the loco firmly as it will fall over if it isn't supported!
Here are pictures of how I stabilized G118X-00V05 while putting the new driveshaft in:

OR: using a pair of Threading Tweezers for a Serger (sewing machine):

(the tweezers themselves:

And here is a picture of the inside of the (old, broken) assembly with both the yoke and the driveshaft inserted into their respective grooves:

The key is to stabilize G118X-00V05 while it is still attached to the center gearbox shaft. This is tricky and will take time. Before you stabilize, turn G118X-00V05 so that the screws are easily accessible while you're working. Don't apply too much pressure with the stabilizing shims as the bolster and the yoke are both old and fragile--only enough pressure to stabilize the bolster while you insert the shaft and then screw the bolster cover back on. It will take time and patience.