Not sure how the Holley TBI works, but based on you say it idles properly when cold and then poorly when warm, I'd check vacuum at cold idle and then hot idle and see what the difference is. Does the Holley system work on maf or map sensor.
After rereading your post, do you have a complete Holley EFI system or a Holley replacement throttle body using the GM ECM. If the latter, then I would definitely guess that you have an intake leak, a bad IAC.
That's my guess. Some more info would be helpful.
Regarding the head gasket, are you using coolant? Are you getting excessive pressure in the cooling system? Anything smelling funny out of the exhaust? Smoking white? Any sludgy residue on the dipstick or oil fill cap? If no to all of those, I would say your headgaskets are fine.