Why is cooling such a mystery?
First off, and I'm not sure if you are implying this by your post, but a lower rated thermostat has nothing to do with what temp your car will operate at. You should be running a good quality 180 or 195 degree thermostat, such as a Robert Shaw.
Second, 200/205 is really not an insane operating temp. Did you change anything with the motor? Add compression? Change cam? Timing changes and a lean AF mix can certainly bump you those few degrees. If you have a newer MP distributor with the Mallory guts you need to tweak it a bit.
Are your hoses passing coolant, not collapsing, and all about the same temp to the touch?
I don't know why everyone insists on running aluminum radiators, but I see a whole lot of posts from people with cooling issues that start out with a "new aluminum radiator." The stock stuff works; when you can, run it.
Ok, how do we fix it. Take the car out and see what happens when you drive on the highway. After about 40mph the fan isn't doing much. If you're doing 50 or so for several minutes and still running hot then you know you have an issue that is not fan related. If the temp DOES drop, then you know that your problem can probably be fixed with, first, properly sealing the rad to the body, possibly adding a shroud, and running a proper fan. What kind of fan are you running? Is it positioned correctly in relation to the radiator? Is it a thermo-clutch type fan, and does it operate properly? Have you changed pulley sizes? Have you changed water pumps, or water pump housings?
Pushing more coolant faster through deeper radiators is not always a cure for cooling issues. In fact, it usually causes a lot of problems. The coolant needs to circulate at an optimum rate to guarantee that it actually has a chance to dissapate heat through the radiator, too fast and it won't work. You can actually also have too deep a radiator, whereas the air flowing through will reheat the coolant.
One more thought for you, but if you installed a new thermo sensor as part of the rebuild, a new harness, new gauge, or simply unblocked a lot of 40+ year old crud from the system, it's possible that your car ALWAYS ran at that temp.
Start off simple, add some water wetter (it DOES work) and burp the system. Review what I wrote and see if you can spot the problem, if it still exists. AND, if the engine does not run hot enough to overheat (actually gushing coolant) or cause detonation/knocking, then it really isn't running hot or overheating, it's just running hotter then what you are used to seeing.
