You could do all that, but you've already got the basics of what you need, so why reinvent the wheel. I use a palmer's stocker tube stick feed on my unibody phantom: Stocker Tube [PPSP051] - $40.00 : Palmers Pursuit Shop!, Where Custom Still Means Something
For a 12 gram dropout/bucket changer, get one that screws right into the back of any ASA. CCI makes them, and they are very common here in the B/S/T areas. Or, look for an old drop-out valve-body, and simply swap out the valve, tube, etc.
Very easy, will keep the originality of the old unibody, and probably a bit more affordable.