In short, use a 91 radiator, and a Comanche overflow bottle. It's a tight fit over the wire harness.
Next is getting the fan to turn on. Either put a manual switch on it and always watch the heat gauge, or fix it. Either use the Speedway adjustable fan switch kit, or rig your own adapter. A 92 thermostat neck will allow you to go crazy trying to spec a 180-200 degree thermal fan switch (not a sensor.) You can't use the 92 up because it's a variable voltage, not on/off. I adapted a Merkur switch, basically a pipe adapter and long wires back into the stock wiring lead. A pigtail lead was about impossible, cut the plastic down and used a ATM fuse holder slipped over the pins. Did I say it looked hillbilly? The Speedway kit lets you control the temp on and override it, too. Very elegant, hooks up to the gauge sensor, avoids the whole mess.
Adapting the old switch into the radiator at the old location is a loser, it's positioned too late in the cycle, and it would take a custom radiator. In brass, possible, not so much in plastic. If that was the deal, we'd just get the necks added.
Get a 92 up heater valve and reconnect the hoses, watch the flow direction or it runs cold. It will take a adapter for one hose to fit the larger core tube. Looks like a nest of clamps.
The closed system works just fine, supply of the turtle tanks is out there. What most don't like is the burping and filling necessary. That still has to happen, the radiator is lower than the highest point in the head. Unless you need to buy a new rad and think you can live with sciencing out two different systems, it's a lot of expense with the only return being a filler neck.
Gotta say it - been there, done that. It's not a project with much return.