the only additional info I can add, is when I initially put together my 360 with the comp XE262, I had an eddy performer 750. it ran like a scalded cat above 3k RPM, but I could never quite tune the low RPM bog out of it, and my car felt kinda soft on the low end, it wouldn't light the tires up romping on it, you'd have to deliberately powerbrake it. I switched to an eddie performer 600, and I gained a ton of low RPM responsiveness, as well as 2" idle vaccuum. stomp on the gas and the rear would light up and start to walk out on you, and if I did lose any high RPM performance, it wasn't noticable.

the huge neutral-drive RPM drop you mentioned initially makes me think there's still something wrong with the timing, or possibly with the torque converter, but I suppose it could be carb tuning.

when you were messing with and checking the timing, you did have the vaccuum advance disconnected and plugged, right?