I just finished putting a new head on my truck and ...

It could have been a number of things.  The mechanical timing might not be right, the rocker arms might not be adjusted right.  The new gasket may be thicker than the old one, lowering your compression.

Do a compression test on your cylinders as that might give you an idea of the problem. 

With good compression, you probably have the proper timing also.  How about your air/fuel mixture?  Running too rich or lean could cause loss of power.  Is your engine carburated or fuel injected?  Did you check for intake manifold leaks?  How about your rocker arm settings?  If they don't travel enough you could get good compression but not enough air or fuel.

I'm still trying to figure this one out.  The intake is not leaking.  Did you check the whole system for vacuum leaks?  Have you checked what ever you have controlling the fuel injection? Have you checked the fuel system pressure and the pressure regulator?

The only things you touched though were the vacuum lines and the rocker arms that we didn't eliminate.  Do you have hydraulic lifters or solid tappets?  Are your rocker arms adjusted hot or cold? This is still a major factor.

Your exhaust could be the problem if you got a sensor dirty or otherwise messed up your EGR system. 

i am ASSUMING you changed the head gasket because you had water going threw the exhaust or for over heating, if that is the case you may have a pluged cat convertor, timing of ,fuel filter pluged, fuel pump,to check for exhaust looosen from pipi at manafold than take for a ride and see if there  is any changes, then check fuel pressure,ignition timming,good luck