To be honest mate there are loads of things that could effect the start up.. i know youve ruled out a lot of them however its prob best to just take the car to a tuner and say 'fix it' chances are they will sort the problem in half an hour!
If the car ran fine before the head gasket change then there is no reason why it shouldnt afterwards.. only the things youve moved should really effect it.. (assuming youve put the head on properly) ie dizzy, and cam timing..
I know things can go wrong at the smae time.. ie fuel ecu, metering head etc but this is very unlikely..
I wouldnt want to go to the hastle of changing the metering head unless i was sure that was the prob..