Change the plugs and check them for heavy black or burnt light brown look. After a short run.
Do you have the accelerator pump in its full on position where the lever has the greatest movment on the pump. That may help if it only help a little it will probably need larger jets.
If you run out first and second up to redline it runs fine? What if you go straight to third at redline in secnd so rpm does not drop will the engine take wot then. Also dont over do it trying just wanted to ask if you have tried it. Dont want to see you hurt it just to see. If wot is ok at high rpm then needles may need to be tweaked. Larger jet will be needed if wot throttle is not working in any gear unless its granny gear trans where third is the first long gear. Also does wot make any power or is it still lame in first and second.
My own lean rich test is simple does it run better when the air is cold or hot. Runs better in the cold then its too rich in the hot its to lean. If the colder it gets the worse it runs then you know its way to lean. Bigger jets or more fuel pressure are in order. Runs better in cold weather then its jetted to rich and fouling the plugs.
And yes timing is easier to check most of the time. I also would not try the holley a second time.