Cooling Issue

Do the gutted stat test.Yours is sticking. Might as well gut that one.

In my stash,I have about 4 different fan pulleys and at least 2 different crank size pulleys,and a big box of spacers.
I have under drive on my 360 too, same size pulleys as you, and it's fine.

Stock stuff works perfect on stock powertrains for it's intended stock-type useage.

When we hotrod stuff, we have to be prepared to un-stock it. We mod the heads, the valve timing, the CRs, the ignition timing, and the fuel system. And we put hi-stalls,and performance rear gears in it. Sometimes we add 125 or more horsepower to it, and ask it to rev 1000 rpm higher.Making power adds heat. It is the heat that makes the power, the heat of altering the engine to burn more gas. More gas equals more heat. More heat equals more power. Mostly when you are in deep, but the inefficiency at low rpm of this now H.O. engine, means it is making heat there too.
Why would we expect the stock cooling system to work.

I can't remember if you stated your cam specs of CR yet, but the stock 340 cooling system is just enough for it's 268* cam, Cr, and TC. I think it can handle 1 or maybe 2 cam sizes and a 2800TC. After that, it needs upgrades. That system is basically a 318 A/C system. When the factory installed A/C in an A-body, they upgraded the rad size, the fan, and the pump.And that was just to handle the compressor and the condenser. So that gives you an idea of the thinking involved.
If you have a 230@050 and a 3000TC, The stock system is long ago inadequate. You are on the right track with the rad, and the 6-blade, and the shroud. You already stated that you have the anti-collapse spring in the lower hose, and plenty of timing. There are only three variables left; the stat, the pump, and the fan-clutch or fan.
The thermostat is by far the cheapest to replace, the easiest to test, and the most likely culprit.