Glad you got it out. I see a few things of note in the pictures.
The drive gear holder has been spinning/slipping in the spindle. I suggest checking your spindle to see if there is any damage (looks like there might be some in the picture). I'm speculating that the previous owner tightened the draw bar VERY tight in an attempt to keep the holder from spinning in the taper. If the 90 degree head is in good shape, I don't see why this holder should slip unless the taper is damaged and the draw bar won't hold it in correctly. I suggest checking things out and deal with any possible issues.
I see there are no keys in the spindle face. They may have been removed because they interfered with the gear when it was inserted in the spindle. These keys will keep holders from spinning when used. I suggest making a pair (or at least one). You might not be able to use them with this drive gear and your overhead, but you can use them with other holders.
Looks like your K&T setup is identical to my B&S. If I remember correctly, the drive gears for the overheads on our Cincinnati mills at work did use drive keys. I don't have an overhead on my B&S. I did mount a Bridgeport M head on it, but don't use that now since I have a Bridgeport 2J head mill and a Bridgeport 2/3rds clone (Jet).
Again, make some room and hang on to that Bridgeport of yours. That type of mill is MUCH more versatile than these big old horizontal mills. For the work I do, hobbyist, small repair items, etc., if I had to pick one machine, I'd get rid of the horizontal mill in a heart beat. But, YMMV depending on the type of work you do. Personally, I'm not too concerned with taking heavier cuts with the bigger machine. I look for versatility and what I can do with a particular machine. But, as long as I have the room, I'll keep my 2B horizontal!
Ted