Occam's Razor, folks. All three plug wires for Bank 2 were installed at incorrect locations on the coil. All I can do is just chalk that up to late nights in the shop and a temporary loss of focus...
After correcting the plug wire situation, she fired up and ran like new. SES light stopped flashing immediately. SES light remained steady on, but after erasing the P0300 code with my reader, the light stayed off. Probably the best news of all, considering this was a head gasket fix: temperature is staying right where it should. Despite my bleeding the coolant system per the manual, the temp gauge needle is 'floating' a little, but not nearly like when the head gasket was gone (bottom of operating range, to red, back to bottom of op range, within no more than 30 seconds at times). I'm hoping that any small amount of air remaining in the system will work itself out through the coolant reservoir (seems to make sense that, as long as air isn't constantly contaminating the cooling system like when my head gasket was bad, that the air should make its way to the reservoir and out). This appears to be the case, as the 'floating' of the temp needle lessened as the first test drive progressed.
After completing a pretty major repair on this beast, I'd like to let the forum know that I'll be glad to field any questions related to the head gasket job, and I will answer them to the best of my ability.
Take care, and thank you to all who have contributed so much valuable input to this forum.