I would start by verifying your timing. Base timing at idle with the vacuum disconnected and port plugged. Rev it up until it stops advancing (around 3000 RPM) and note the timing. Connect the vacuum advance and see how much timing it adds. Not sure how to check the amount of vacuum advance when using the ported vacuum port, I always use the manifold port, but that's a whole nother discussion. You can use a hand vacuum pump on the vacuum advance also to see how much it adds. On the old school SBC, you want about 36* all in (base + mechanical, no vacuum). I think this is different for the Vortec heads, but I am not positive.
I wonder if your spark plug gap is too large and can't handle the increased load. I think for HEI the gap is 0.045, but you might try closer to 0.035. I would stick with copper plugs also, none of that iridium or platinum.
You might be able to leave the timing light hooked up as you drive to verify if it stops sparking.
If you still have the original exhaust with oxygen sensors, you can read those. They operate on a 0-1 volt scale. You can connect them to a narrowband air fuel gauge or possibly use a multimeter. Since it sounds like the ECU is still hooked up, you might be able to read it off the OBD2 port. A wideband would be MUCH better, as they read more accurately over a wider range.
Not familiar with your carb. Might it be that it is fine on the primary bores, but has issues on the secondary bores? Is there a way to easily disable the secondaries and try to go WOT? Does it react the same if you slowly open the throttle vs quickly open the throttle?
