OK, several more things to consider..........
"Jerking me forward and then back. From idle to 4K, no problems. "
This still sounds to me like a fuel/carb problem as you hit higher RPM's and the engine demands more fuel. When the carb runs out of fuel, it will nose over, then the fuel will catch up and slightly fill the carb bowl, pick-up power/rpm's, and then nose over again once the fuel has been used up again. Using the Sea Foam you may have dislodged something and it got lodged somewhere in the carb internals. Not saying I am right, but it could be a possibility at this point. BUT....
Those plugs look bad, but it could be excess oil, carb running rich, or plugs not firing correctly. The R43S is a "cold" plugs and may not be the correct ones for your application. You might want to try a "hotter" plug like the R44S or possibly the R45S. The R43S is the safer bet only because fouling is a much more desirable "problem" to have than detonation. Fouling can cause misfires or poor running and a hotter plug may be necessary. If you're running too rich or burning some oil, the hotter plug can sometimes compensate for this as long as your engine isn't knocking or pinging away on the road under load
. If the engine does begin to knock or ping on you and you cannot compensate for this through a small timing adjustment, then you will need to stick with the R43S. So you may need a hotter plug to help burn better. Try the R44S plug instead and this may eliminate all the sooty looking appearances of the plug and may help with your problem.
.035" - 0.40" gap is fine. I prefer a tighter gap at .035"
You did not tell us what type of distributor
you are using? With the engine losing power near 4,000 RPM's, the plugs fouled, ...........might it be an HEI or aftermarket electronic ignition? If so, these units need a 12volt power wire. The factory ignition wire to the original points type distributor used a resistor wire to supply about 7 volts to the points so they do not burn. If a 12volt wire has not been used to jump/eliminate the factory resistor wire, you will not get enough voltage
for the electronic ignition to work/fire properly and you would have the same problems you describe.
I assume you have a good coil as these can go bad? Anything with regards to ignition may cause a poor spark at higher RPM's. A bad MSD can't be ruled out. Can you change your wiring around to eliminate the MSD from the ignition system just to test it?
Other things to check would be the 5/7 plug wires. Have read that if they cross on top of each other (touch), they can cause problems.
Fuel pump could be pumping gas just fine at lower RPM's, but if something were amiss with the diaphragm, it is possible it could be causing issues. Is it a fuel pump made to work with the ethanol gas? Only way to really know would be a fuel pressure gauge. If low fuel pressure, then you can back track from the carb. Fuel pump, the sock https://www.opgi.com/chevelle/CH28637/
on the end of the fuel pick-up collapsed or plugged up, or something floating around in the gas tank.