I recently got my T/A out for the summer. It has a goodwrench 350 with about 50k miles. Edelbrock carb, hei dizzy, headers, etc. After its annual tune up i took it out and it ran excellently. After about 50 miles I did a short half throttle pull, and it ran great, but as soon as I lifted it started to stumble and cough. Now it seems as if its running on 6 cylinders until i put it under a load, then it runs perfectly.

After I replaced the fuel filter, plugs, wires, hei vacuum advance can (which wouldn't hold vacuum), checked the cap and rotor, retuned the idle screws on the carb and it ran perfectly again. Drove another 150 miles with no problems, then tried another half throttle pull and bam same problem again. Barely runs unless I put a load on it, then it runs like there's no issue. Seems like an ignition problem to me, but unless something is wonky inside the electronics of the dizzy itself, i have no ideas.

I completely stumped. Anyone have any ideas or suggestions? The engine is in great shape (good compression, burns no oil, etc) and ran perfectly last year.

A few things to check:
vacuum leak, water in the fuel, are the weights & springs under the rotor clean and moving freely?

Could be water in the fuel, I have about 1/4 tank left out of the full tank it had from storage. Springs are moving freely and I can't find vac leaks anywhere.

Could be any number of issues, including those listed above.

BUT--the GM Goodwrench engines came with the absolutely cheapest POS timing sets available at the time. A sub-stock-quality timing set running a high profile camshaft won't last too long without stretching the chain. I'd check the slack in your timing chain. My bet that it's your issue, and it may have even jumped a tooth. Under load, the timing gears take up the slack on one side only, and your timing may be close when this occurs. With no load there is very likely slack on both sides, making your timing erratic.

Checking the slack is very easy conceptually. You'll rock the rotating assembly back and forth using a breaker bar and a 5/8" socket on the harmonic balancer bolt. While doing so, you'll observe the movement of the rotor in the distributor. Ideally, you'll want to move it one direction until the rotor moves, then stop. Turn the engine in the opposite direction and stop as soon as you see motion in the rotor. Record the number of degrees you turned the crank without movement of the rotor. Anything more than about 5 degrees and I'd replace the timing assembly.

Problem is, there is a bunch of stuff in the way, and getting the 5/8" deep well socket on the bolt with the stock fan in place (and keeping it there) is a PITA. Also, many of the SBC harmonic balancer bolts tend to loosen, rather than rotate the engines rotating assembly. Best thing you can do if you can is to remove the fan to keep it out of the way, and remove all your spark plugs in order to make turning the engine over easier.

Not the timing chain. The cam takes the same force to turn at any given RPM no matter how much power the engine is producing.

Which is NOT to say "the chain is fine" or ANY SUCH thing; only, that it CANNOT cause the symptom at hand.

Check the rotor tip for being turned to the side.

Put a timing light on it. I had mine set, drove it up the street and back and when I stopped it coughed and died. Turns out the distributor moved, bolt wasn't tight enough. Set it back at 14deg, tightened the hold down and it's good to go yet again.