How about putting the yokes back on and where the U clamps hold the u-joint drill a flat plate to fit across that end of the yoke and bolt it to the yoke using the 4 holes. Then drill a 3/8 hole in the centre of the plate, put a 3/8 bolt through and weld or lock on with nuts. Now chuck your drill on the end of the 3/8 bolt and go at it.
Or get a piece of tubing that just slides over the shafts, support the shafts with blocking underneath (so you don't stress the bearings) and with a round end punch pound the wall of the tubing into the valley of the splines so you form inner splines on the tubing. Next go to your reject tool bin where you keep those chi sockets you get when you buy a used tool lot, find ones that fit inside the tubes and braze or fasten them in. Now you've got some special tools for the task. As long as you're not trying to turn the whole drive line with your drill they should hold up.