Twenty minutes? With a five minute warmup and the silly pedal boarded for twenty minutes, the thermostat is probably just barely starting to "crack"... Slow flow (barely cracked thermostat) absolutely should make the top tank much warmer than the lower tank.
Overflow from the overflow tube is normal until the coolant is at the right level. On flat ground, looking down the filler neck you should just barely be able to see the top of the coolant hiding around the corner. Any more than that will burp it's self out through the tube, or all over your compressor and left fender if the overflow tube is plugged.
If the top is truly hot, the engine is truly hot, and coolant truly should be flowing, you can't blame the thermostat for the top tank being hot. Statistically if they have not been serviced for a while, the lower hose and lower half of the radiator (and in bad cases the lower half of the tubes) are all packed with silt. Flush products are not appropriate for this. That would be like trying to clean up spilled marbles from your living room floor with carpet shampoo. It won't work, you'll get shiny marbles but your living room is still a mess. The solution is to simply pick them up. The solution for silt of a magnitude that will actually restrict a passage is not "flushing", but actual removal. Removing the lower hose is the easiest (and yes, it is thoroughly and completely awkward, no free lunch here...). Garden hose straight down the filler neck, that'll get a decent flow in the right direction in the lower tank. If you want to check this, open the radiator petcock, as it's draining squeeze that lower hose around a little bit and see what color the coolant turns that's running out the drain fitting...
If you find copious amounts of silt in the radiator, it's prolly at various places in the block too. Remove what you need to, get good garden hose flow throughout the engine (including removing the engine petcock for the largest possible opening). The garden hose will get most of "the big stuff".
Then proceed with the chemical contraptions if the condition inside the coolant passages warrants it. TMs have info on this.
