Slant stumbling on acceleration

Ok this one's starting to get to me. Hang on, this will be a lengthy run-down:

I must preface this with the fact that my knowledge of the history of the PO's work (or lack thereof) on this car is basically nil.

So a while back I start noticing a very 'slight' stumble on acceleration. Nothing major and only occasionally, so I don't think much of it. Over time it gradually becomes more noticeable. It's about this time I find that I start having a flooding condition and realize the float in the 1920 carb is probably a goner. Seeing that the replacement costs for the 1920 float is pretty steep, I find a recently rebuilt (by owner, not rebuild house) 1920 on ebay for about the price of a new float. So I swap it out. Flooding condition resolved!

But the stumble remained? Hmm. This made me think it wasn't carb related. So I replace the fuel filter in case it was clogged. The Wix metal filter I pulled off looked pretty clean. That didn't fix it.

So I start thinking vacuum advance. I see that the old vacuum advance hose is in pretty rough shape and looked pretty worn down by the distributor. No problem. That's a cheap/easy replacement that probably should've been done, just like the fuel filter. Nope, that didn't fix it.

Ok, that vacuum advance pod looks WAY old and caked in oil over the years. Swap in a new vacuum advance pod. That doesn't do it either.

Now I'm scratching my head. The stumble I have is not at very low throttle. It's if I start to punch it a little that it bogs and wants to die. Opening it up further to WOT will sometimes get it past the bog. It idles just fine and I can punch the accelerator in Neutral and there is no bog. Only under load.

The condition is worsening this entire time. Only now I start to notice a wrinkle I hadn't noticed before: It's only happening after a couple minutes after warming up a little. Hmm. Temp-related? I look at the coil and it looks as worn as the vacuum advance. I look up the part number on the side and it's some old GM and tractor coil from the 60s or 70s. My multimeter doesn't have batteries to test it but I figure what the hell and replace the coil. Nope. That doesn't do it.

So that's where I'm currently at. Basically stock 225 with Holley 1920. Will pull the plugs this weekend and take a look at their condition and also check compression while I'm at it.
Any ideas where to look next?

- Timing? I don't have a timing light but will try to get my hands on one. If the timing has become retarded, wouldn't I have issues at other times besides just part-throttle?
- Still the carb? I had originally eliminated this when I swapped to the second 1920 with no apparent change, but now that I see that it's somewhat temperature-related I'm still wondering about this. I should note that the issue seems to be there regardless of the mixture screw position, 1.5 turns, 3.5 turns, same issue. Float position looked correct on the second 1920 carb. There is a nice healthy shot of fuel on the accelerator pump.
- Valves? I adjusted the valves (.10, .20) only about 500 miles ago.
- Vacuum leak? Didn't see/hear anything noticeable. Sprayed cleaner around the carb base with no change. Can't feel anything.

I'm in the process of getting my super six conversion together so if it's just the carb then I'm inclined not to spend the effort farting with the 1920 extensively since I'll just be abandoning it soon. Then again, if it's something else I want to make sure it's resolved before I do the conversion.