So a week and a half later, the parts and the free time to do the work have materialized, and I'm immediately confounded:
I've unbolted the set of rocker arm towers (set of three for cylinders nos. 3, 4, and 5) and they appear to be very firmly set in place, despite barring the engine all the way around and checking for the optimal position. The loosest place I can find (visually) is with the No. 3 cylinder exhaust valve at 12:00, and it seems that this
is the right place, but is there a trick to getting them out that I'm missing? The only bolts I've removed are the three hex-heads on the rocker towers; the XZN ones at the bases appear to be head bolts and not related to the rockers at all.