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Sounds as though you've got yourself an interesting tractor. Given what you're describing, the first thing I'd do (with the engine off) is to pull the cover off of the governor (in front of the carb) and see what it's doing when you adjust the throttle setting - that's where the engine speed is controlled. The first third of the throttle shouldn't stretch the governor spring (the tractor is working off of the idle circuit); the rest of the travel you should see it stretch. It's pretty common for the throttle linkages to be sloppy on these old tractors, and you may have to do whatever to tighten it up - but I doubt that's all of your problem. It sounds as though your governor isn't working, and that would explain why bad things happen under load (the governor isn't supplying enough gas to handle it). The governor does its work by adjusting (very quickly) the position of a butterfly valve between the body of the carb and the intake manifold - it can get bound up either by wear or if somebody doesn't install the carb correctly. If you don't see anything obvious in the governor, I'd think seriously about checking the linkage between it and this valve. It would be interesting to know how fast your single-speed engine is turning at this point (is the tach working?) and what a timing light is telling you. You should have either 3 or 4 timing marks on the fan belt pulley which line up with a pointer off the front cover which you should be able to see if you look down from the right side of the tractor near the oil fill. The timing should line up with either the mark which is isolated from the others (TDC) at low idle speed, or with the second one from the opposite side (22 degree advance) at high idle. If you're somewhere in between, it probably should be closer to the high-idle mark. You probably should check the distributor to see if the plug wires are in the proper order - firing order is stamped on the engine. Hope this helps. Let us know what you find.
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