17 hours ago, jpc3006 said:

Dadsun,

FYI, when I replaced the flat-tops on my '73 with round-tops from a '71 many, many years ago I bought a new thermostat housing and the metal coolant tube that runs around the back of the head from the intake manifold for a 1971. This eliminates the part you call the "sensor" altogether and routes the coolant from the thermostat housing through the intake manifolds and back to the water pump.  Using the 1971 parts also eliminated the need to plug the extra connections as there is no provision for coolant being supplied to these carbs. I have run the car this way for over 35 years and have had no problems; so my answer to your 

"QUESTION: Can I just get rid of this 5/8" hose and sensor altogether for round tops? That would only leave the coolant flowing through the intake manifold on that side of the engine compartment." 

is yes, if you are installing 1971 round-tops as that is the stock configuration. 

This is exactly what I'm in the process of doing.  I came across an old thread where @siteunseen had the very coolant hardline I needed if I wanted to avoid having to plug a bunch of unwanted ports.  I was also able to take the old manifold coolant connector off of my flat top setup and screw it into the new thermostat housing I ordered.  It was a much smaller threaded opening than in my old thermostat. Pic:

 

All I think I need now is the coolant hose reducer segment to go from 3/4" to 1/2" on the passenger side of the engine. Checking my local industrial parts store. Pic:

I'm glad to see you have that "Y" pipe on the passenger's side. Mine is on my car so I wasn't able to offer that. Looks like you have a good plan. Glad I could help out the small way I did.