It will be a miracle if i ever finish this... with every day comes a new problem

Drilled and tapped the new key for M5 but cant find any bolts long enough. (125mm). Would studding be ok? The other option is to make up a small drawbar 8mm dia. to go thro the chuck then turn down the end for an M5 thread, problem is making it strong enough and providing some sort of slot in the head for tightening.

The next challenge is deciding wether to cut down the main post and fix the head at a set height or allow it move up and down to suit the job, like a proper mill I will need to sort out a better clamp than the existing grub screw arrangement , may be cut a slot and then bolt up to pinch the main post? would this help remove the current play, and provide an accurate reposition when moved?