If it's dropping that low (which is too low) on its own, it's sounds like you're boiling water in there, so she is runnin' too hot. Could be the radiator core, could be an excess of sediment in the block or head, or could just be runnin' too hot.
If they've been left to make sediment, it tends to pile up at the rear. One motor like yours I tore down had sediment up against the back wall that ran probably two-thirds of the way up the back of the #4 sleeve and sloped down to surround #4 about 1/3 at the front and still cover a good bit around the bottom of #3. I'm talkin' hard, dense sludge. Similar up in the head.
Apart from keepin' #3 and #4 from being properly cooled, there was enough sludge in there that it also significantly reduced the liquid capacity of the whole cooling system.
Three things come to mind.
1) your radiator is clogged up as you suggest
2) you have enough sediment in the crankcase and head that you can't hold enough coolant to do the job, or
3) your timing is advanced far enough to make you run so hot that even a clean cooling system can't keep up with it.
To identify and solve the problem, I'd approach those in reverse order.
I've had perfectly clean cooling systems boil over from bad timing (an embarrassing rudimentary mistake on my part that I won't admit to!), so check your timing first. If that's okay (like within 5* of being right, that's not your problem, but it wouldn't hurt to adjust it back to TDC while you're at it.
If your timing was reasonably okay, then it's time to look at the cooling sytem. Fill your system with water to halfway between the top of the core and bottom of the filler neck. Then drain the system into something you can measure -- wash pails with measurements on the side, a five gallon pail and pour it off through a funnel into gallon jugs -- whatever. The system holds about 13 quarts. If you get much less than that, you've got a sludge problem in the block and head. All the usual cautions about disposal of any anti-freeze in the drained mix still apply.
If you can get 12 quarts or so out of that exercise, though, your sludge problem is minor and we're back to the radiator.
For minor sludge or core blockage, I'd first try flushing it. Use washing soda. Arm and Hammer makes it, but it's sodium carbonate, NOT bicarbonate. It's usually up on the top shelves in the laundry section of the supermarket, right next to the 20-Mule Team Borax. Mix two pounds of that up in pails with the same amount of water you got out earlier, pour it into the tractor, then run her hard and hot. Maybe even run her hard and hot and long, or two or three times with cool-downs in between, then drain and see what you get out. Guar-ron-teed, it will be brown and ugly.
At the very least, this ought to scrub out your radiator core some. Several runs might also help break up and wash loose any sludge in the block if that's a problem.
Good thing is it's summer, so you can follow up with a couple of changes of straight water just to clean things up, or even another dose or two of washing soda before a last, hot run or two with water before switching back to an anti-freeze mix for winter.
If the volume of sludge in the block is really severe, then that's something else.
The thermosiphon was actually a pretty efficient system. If you ever heard one of those old girls working from a distance, you'd only hear the exhaust when the governor took over -- when they were runnin' steady all you heard was the howl of the fan. They're made to draw a lot of air through an adequate radiator. When they're working right, the fins at the top of the radiator will be hotter than you want to rest your hand on, but the bottom will be only warm or even cool to the touch.
Bottom line is you're boiling coolant away. You have either reduced flow (radiator blockage and/or severe sludge), reduced capacity (sludge), or you're running too hot in the first place (timing, most likely, or other problem).
Again, I've gotten too long-winded, but hope I've helped.
