2" of vacuum at RPM, with a huge carb, certainly seems to indicate that the heads are wanting more! Me thinks a tunnel ram is in your future!
You didn't mention intake vacuum with the 8 barrel intake. Was it high also? I would assume it dropped some with the 8 barrel. If not much, then it seems like the heads want more than the intakes can produce. Certainly on that 4 barrel, at least. Were the 8 barrel carbs a known variable? In other words, had they been used successfully on another 8 barrel engine? If so, there has to be a reason they were so pig rich. Vacuum alone shouldn't do that, unless, combined with airflow, it started pulling fuel from places it shouldn't have.
It looks like you have temp probes in each exhaust tube. How were the exhaust temps? That could indicate any issues with intake flow problems (reversion issues with such a big cam, or just a bottleneck?), or any coil/ignition issues, if there are any.
At the risk of sounding like a doofus for missing something obvious, how does that center shaft bolt work? Is the bottom of the intake rocker slotted so that the bolt travels through the rocker, and the rocker pivots over the top of the bolt?
On the intake rocker adjuster; Is there not enough room to move the adjuster over slightly, for additional strength, and modify/machine the intake adapter for some additional pushrod clearance? Maybe there's not enough room around the port to do that?
Okay, sorry for all the questions. Thanks for the great write-up, and it seems like you're only scratching the surface with power production. This is going to get really interesting, as I think you have some real intake development in your future.